What Jim Jones had going for him was the fact that he gave people life. Drug addicts, gang members, and all sorts of people joined with him. Jones was also regarded to be 'black' despite being a white man. In short, he really connected with the minorities and manipulated them by having them give up all possessions to the church. Many members lived in a home created by the church and put all money into the church in exchange for being taken care of. For example, you pay all your money and if you need a dentist appointment it's all cared for.

There wasn't any real big event that caused people to flock to the Peoples Temple, rather it was Jones' charismatic personality and manipulative tactics that charmed people. He was also a very influential man, meeting with celebrities and governors. There's a really good documentary you can watch here that will go more in depth if why people joined and how the dreaded day played out where almost 1000 people were killed. Having spoken to a survivor, I can assert that many people who had none were given 'life' by Jim Jones and that is why they joined. As to why the members didn't leave, this paper explains the mentality of Jonestown and what really happened perfectly. I recommend checking out the entire website.

Also, for /u/WoodChucking

Peoples Temple was a congregation that started off large but the group that went to Jonestown was only a small fraction of the actual group. Many people became disinterested and left due to Jim Jones being labeled by the media as a fraud and the reason why Jones went to Jonestown was to prevent his members from hearing the truth to be published in the papers about his fake miracles and other false practices. I don't have an exact number, but the total death count was between 990 and 1000, if that gives you an idea of People's Temple in Jonestown vs. in California.

I don't have a ton of time, but I wanted to interject here to say a few things. (Not as much necessarily to you as to the thread and anyone reading it.)

First, you are asking an excellent, historical question. You ask not just "why did people join Jim Jones?" but "why THEN would people be ripe to follow someone like Jim Jones?" Unfortunately, the answer from /u/UnhackableWaffle, while it seems accurate (to the best of my knowledge) and informed (if under-cited to this academic's taste), is not a historical answer. The basis of history is context, which that answer elides. Being welcoming to minorities is only remarkable if the context is provided that minorities felt under attack in this time, etc. I say this not to poke at what is, indeed, a factual answer, but to explain how even "correct" (as in, true to the facts as we know and understand them) answer can still fail to be historical.

Second, I want to try to answer your actual question, but because my time is somewhat limited, I'm going to make this all bullet-pointed and might be coming back later to flesh out my own citations.

Crucial context for the rise of any/all forms of "alternative" culture in the 1970s is that in the United States everything was falling apart in mainstream politics, economics, and society -- at least from the viewpoint of mainstream Americans. When I teach the 1970s to undergraduates in the American history seminar, I often have them debate which of the many crises of the 1970s in the U.S. was the most important historically, and I get a variety of answers, any number of which have earned As, because you can argue for a lot of different events with compelling evidence. Consider this laundry list:

• A full-on constitutional crises as a President resigns from office in the first time in history. AKA Nixon and Watergate, the scandal so epic that all other political scandals get the suffix "-gate" attached to them. Nixon abused the power of the CIA, the FBI, and the IRS to undermine his political opponents and even signed off on breaking into the Democratic National Committee's headquarters in the Watergate Complex, and then flagrantly breaking oh-so-many laws in highly illegal cover-ups of these and other actions. 69 high ranking officials were indicted and a President resigned. Seriously, when you read back to the hand-wringing of the era, it does somewhat put today's hyperbolic rhetoric about the end of American democracy into perspective. America survived Nixon, which I do take heart in. (The parallels between Nixon and Trump are actually fascinating in terms of party politics, relationship with the media, and, ultimately, corruption, but that really isn't answering your question. Citations coming here.)
• The Vietnam war inches ever closer to 20 years on ongoing fighting, as millions of people die, tens of thousands of them being young Americans subject to a draft, with those that survive often coming home with addictions to heroine and alcohol, even before the PTSD sets in. This all in the context of increasing war-weariness in the aftermath of Korea, the Yom Kippur War and other messy entanglements with Israel, the ongoing Cold War with the USSR and more.
• the 1973 Oil Crisis as OPEC proclaims an embargo, followed by another oil "shock" in 1979. As the price of oil per barrel quadruples from 1973 to 1974, the economy goes into crisis. Think of how many people in the US needed (and today still need) gas in their cars to make it just to work or the grocery store (we've never been that big on public transit) -- and then think of all the competing claims of petrochemical and other industries who need that gas too. The basic cost of living escalates, and inflation skyrockets, even as people waste more and more time waiting in lines for gas stations with ever-higher prices -- and often they close for lack of gas before everyone is served.
• Elsewhere in the economy, Nixon formally ends the Gold Standard, unions are crushed across the country, neo-Keynesianism and neoliberal economic policies come to the fore, and lives and careers are lost in a tumultuous transition economically "Trading Factories for Finance" to quote the words of historian Judith Stein.
• Socially, unrest abounds. Think of the Kent State Massacre and Jackson State Shootings, where student protestors were gunned down in 1970. Youth are so agitated wanting political representation that in 1971, the 26th amendment is passed lowering the voting age to 21. The civil rights struggle continues, hurt and increasingly angry at the length of their struggle and the assassinations of MLK in 1968 and Malcolm X in 1965; the 1970s are the height of the radical Black Panther Party and other nationalist African American movements. In LGBTQ activism, remember that the 1970s are framed by the Stonewall Riots in 1969 and the assassination of the first openly gay elected official in the US -- Harvey Milk, Mayor of San Fransisco is gunned down in 1978. Environmental crises are also unfolding before American's eyes - -the first Earth Day and the founding of the EPA both happen in 1970, and throughout the 1970s Americans grapple with toxic pollution, water, air, wildlife, soil quality, and a trash disposal problem (running out of landfills!) more than they ever had before, as new science reveals new dangers. It is in the 1970s that leaded gasoline starts to be phased out in light of overwhelming evidence of its harm, for just one example.

All of this is to make very understandable the waves and waves of young people who increasingly rejected mainstream American politics, economics, and social patterns out of reaction and disgust to the current events unfolding around them. People were looking for something different, whether through back-to-the-land communes, "turn on, tune in, drop out" escapism into drugs and youth culture (a phrase coined by Timothy Leary in 1967), or even religious cults. Along with the "cults" we recognize today like the People's Temple, one should be mindful of the more mainstream turn to religion to solve social ills through Christian Evangelicalism, tent revivals, speaking in tongues, and multiplication movements like Historic Adventism. So in addition to the particulars of Jim Jone's charisma and appeal, hopefully you can now see more widely why people wanted a panacea, a messiah figure to save them from the ills, injustices, and evils of the world, why dropping out of the mainstream into an isolated and self-contained society was attractive, instead of the opposite. Hope this helps!

Misc Sources Informing My Knowledge of the 1970s (not a definitive scholarly list, just what immediately available in my citation software):

Judith Stein, Pivotal Decade: How the United States Traded Factories for Finance in the Seventies (New Haven [Conn.]: Yale University Press, 2010).

Melani McAlister, Epic Encounters: Culture, Media, and U.S. Interests in the Middle East, 1945-2000 (Berkeley: University of California Press, 2001).

Jefferson Cowie, Stayin’ Alive: The 1970s and the Last Days of the Working Class (New York: New Press, 2010).

Kim Phillips-Fein, Invisible Hands: The Making of the Conservative Movement from the New Deal to Reagan (New York: W. W. Norton & Company, 2009).

Margot Canaday, The Straight State: Sexuality and Citizenship in Twentieth-Century America (Princeton, N.J.: Princeton University Press, 2009).

Gary Gerstle, American Crucible: Race and Nation in the Twentieth Century (Princeton: Princeton University Press, 2001).

Adam Rome, The Genius of Earth Day: How a 1970 Teach-in Unexpectedly Made the First Green Generation (New York: Hill and Wang, 2013).

Andrew G. Kirk, Counterculture Green: The Whole Earth Catalog and American Environmentalism (Lawrence, Kan.: University Press of Kansas, 2007). [talks about growth of druggie and environmentalist drop-out movements esp.]

Robert Gottlieb, Forcing the Spring: The Transformation of the American Environmental Movement (Washington, D.C.: Island Press, 1993).

Martin V Melosi, Garbage in the Cities: Refuse, Reform, and the Environment : 1880-1980 (College Station, TX: Texas A&M University Press, 1981).

Related to this, the Wikiedia article about the Peoples Temple says it had 20K members, but lists 3000-5000 in the side bar summery.

How many members did it have?