There is poorly developed pablic transportation system in Guam. However well developed road infrastructure, thus you can try your luck hitchhiking.
Hitchhiking works quite well at the day time but can be difficult ordeal when it's already dark.
There are few places where is possible to busk. One is during Chamorro villlage night market every Wednesday.
Another good location with plenty of foot traffic at the evenings is in Tumon around Under Water World, however written permission from management is required.