This is a simple one and there is two ways to unlock this:
1.You can either unlock all the SSX team while you play on World Tour and taking on the Deadly Descents. (Doing it this way will save your credits you earn.)
2.Or you can go into Explorer mode and purchase all the SSX riders using the credits you earn after each race. (you will need about 125,000 - 140,000 approximate.)
*You dont have buy Griff, since hes not part of SSX team. Thanks to DrCanadianNinja*