I think they choose people from a list of owners of target brands/models they are testing. I've been picked for owning a 7 series, X series, and even for my Ford F series and my ATVs. They make you sign a confidentiality agreement and no picuture taking is allowed (they made me lock up my cell phone in a locker even). These tests may or may not involve test drives. It depends. Usually it involves opinion on styling and features you like/dislike. I don't know how you can sign up, but once you get on the list, they will likely call you for another test later.

__________________
'06 X5 4.8is, TiAg/black, cold weather pkg., comfort seats
'07 750Li, TiAg/black, sport pkg.
'05 Ford F350 silver met./slate gray, Lariat, 6.0 PSD, CC, DRW, FX4