As a Freelancer, technically you never join the ranks of a particular faction. I assume you mean 'become on outcast' in the way that when people select your ship, it says "Outcasts - *name*"? In that case, I think you just have to keep doing loads and loads of missions for the group in question.

yes, that only works on multiplayer though, and you still will be scanned as "Freelancer Alpha X-X"

Is there a rehabitation center around here? My friend sucked c*ck for freelancer!!!