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.