It's really simple to be friends with the whole neighborhood, just watch these steps:

1.Buy a guitar then play it to learn how to play guitar.
2.Keep playing until you reach 5/10 level which gives you the ability to "play for tips"
3.When you have the ability, go to the park with the guitar in your inventory.
4.When you are in the park, click on the guitar in your inventory and select "play for tips"

People watching you will become your friends if you keep playing. I guess the more your 
guitar skill is, faster they become friends of you.