Yes, because many people who were recruited for the project could not be told what it was until they signed on the dotted line. Security concerns meant most people didn't know what they were signing up for, only that it was crucial to winning the war.

Stephane Groueff, in Manhattan Project: The Untold Story of the Making of the Atomic Bomb, describes how one of the Manhattan higher-ups tried to recruit a friend who was working on radar and the proximity fuse at the Massachusetts Institute of Technology.

The friend declined and said something to the effect of, "Look, friend, I have an inkling you're working on something related to nuclear energy. Can't you wait until the war is over? There'll be time for that later."

Remember, physicists were in high demand for all kinds of projects, not the least of which was radar.

The one person who actually left the project mid-way into it, who knew what it was about, was Joseph Rotblat, a Polish physicist who was part of the UK delegation. The story goes that after the Germans were defeated he decided the reason for making the bomb was done, and went back to Europe. The reality is apparently a little more tricky, according to his biographer: Rotblat wanted to get back to Europe to find out what had happened to his family back in Poland, and his Polish nationality made him a tricky thing for the Manhattan Project security people in any event. I've written up more on this here. Rotblat later went on to be an important anti-nuclear spokesperson.

(Many people left the project during the war without knowing what it was about. Ultimately around 17% of the workers quit per month because the working conditions were poor or for other reasons, but nearly all of them had zero inkling what it was they were making.)

As an aside, there seems to be an implication in the question that not joining would somehow carry some kind of negative weight later on. This isn't the case. There was plenty of war work to be done for scientists other than on the bomb project, and the project did not suffer for lack of talent. I can't think even of cases of scientists who did no war work at all coming under fire for it later. It just wasn't the attitude at the time, perhaps because the US was so flush in scientific manpower as it was. Even people who refused to work on nuclear weapons after the fact of them was not secret (e.g. in the Cold War) did not suffer recriminations, personal or professional.

Paul Dirac refused an invitation to work on the Manhattan project. He was a very peculiar man and didn't want to leave Cambridge. After the war it had very little consequence for him and he ended up working as a professor in the USA.

Sorry if this is off topic but evidently my great uncle worked on the manhattan project, it's published in his obituary. I'm just wondering if that kind of information would be given out, or how possible it is he did?