Some of them will walk up to you and ask for some change. Press L1 to give them $10.
In Algonquin, there are musicians located around the city. When you hear the music start to play, look for someone playing the saxophone. Upon approaching him, the L1 button prompt will appear on your screen to donate $10 to the musician.
Algonquin is the middle island. I assume you haven't gotten that far in the game yet. But just walk around anywhere, especially in poor neighborhoods. When you're standing still, one will walk up to you and ask.