Unlock Beat
Win against Beat in the street race and he'll join you.
Unlock Boogie
To unlock Boogie simply meet her in Kibogaoka Hill.
Boogie was formerly known as Sugar (JSR), and Piranha (JGR).
Unlock Clutch
Find Clutch via DJ Professor K's instructions and he'll join you.
Unlock Combo
Finish Combo's challenge and he'll join you.
Unlock Garam
Complete Garam's challenges for him to join you.
Unlock Rhyth
After finding Rhyth on top of the Poison Jam warehouse in Rokkaku-dai Heights, follow her around. After following and finding her two more times, she'll join you.
Rhyth is Mew (JGR) / Bis (JSR).
Get Suga
Go to Chou street and find 32 spray can with clutch then go to the headless dinosaur jump and land.
After that go to the botton and you will see a girl dancing, talk to her then play againts her in city rush.