Since the Guadalcanal Campaign was just six months after the attack on Peal Harbor, did they have time to train a bunch of new volunteers before that battle? Also, the wikipedia page says US strength was 60,000 but I didn't think the Marines were that big before the war.
The US Marine Corps was indeed very small before WWII. There were only 19,432 Marines on active duty on July 1, 1939. When WWII started but the USA was not yet involved, there was a frenzy of voluntary enlistments into the armed forces. The anticipated active duty strength of the Marines was increased to 38,000 in September 1941, and then to 79,757 for the upcoming fiscal year 1942. These figures proved pitifully small, and enlistments skyrocketed to the point where a figure of 223,000 anticipated enlistments by the end of 1941 alone needed to be approved by President Roosevelt on the 16th of July 1941.
On to your question
All of the Marines that fought during the battle of Guadalcanal (August 7, 1942 – February 9, 1943) would have voluntarily enlisted into the Marine Corps both pre- and post-Pearl Harbor due to the branch's policy of voluntary enlistments before 1 February 1943. Training pre-December 1941 was seven weeks long, then five weeks in January 1942, six weeks in February of 1942, and then back to seven weeks in March 1942 where it remained until 1944. This was plenty of time to train new enlistees, although the timeframe for becoming acclimated and shipping out to their units prior to the fighting would have been very tight if you enlisted in the spring of 1942 instead of immediately before or after Pearl Harbor.
The Marines were notorious pre-WWII for being extremely selective;
During the peacetime years before 1939, Marine recruiting was on a voluntary and highly selective basis. Because of the reputation of the Corps as a fighting force and the very small quotas to be filled, many more men applied than could be taken. Out of 36,356 applicants during 1939, for instance, on 5,861 could actually be enlisted.
President Roosevelt ended voluntary enlistment into the armed forces and increased the draft age via executive order on December 5, 1942. The Marines as a result complied, ending voluntary enlistments and following Selective Service regulations by issuing their first draft call. The battle on Guadalcanal ended on February 9, 1943, and the Marine training schedule was seven weeks long from March 1942 to early 1944. As a result, no drafted Marines completed their training in time to serve on Guadalcanal.
Sources:
Administration of the Navy Department in World War II: Chapter XIV: The United States Marine Corps, by Rear Admiral Julian H. Furer, USN (ret)
http://ibiblio.org/hyperwar/USN/Admin-Hist/USN-Admin/USN-Admin-14.html
US Marine Corps Recruit Training Schedules
In general the First Marine Division was very raw before the start of Operation Watchtower, and the invasion of Guadalcanal and the surrounding islands.
Most of the Division was either being consolidated from prewar units already in the Pacific (5th Marines) or was shipped out straight from the states and training in Pendleton and Lejune (1st and 7th Marines).
Vandergrift, whom had gone ahead with most of his staff and some advance units to New Zealand in June 1942 to help shore up the critical Fiji Route from the US to Australia, was told by Admiral King that the 1st Marine Division would have about 6 months of additional seasoning and training before being used. However desire to strike back after the gain of breathing space at Midway and word that the airfield on Guadalcanal was almost done changed things. In stead of 6 months it was 5 weeks for the division to arrive conduct rudimentary training and then 8 months into the war and just a week after King's initial deadline they were wading ashore on Guadalcanal.