As a 5, I've raced against members of Radio Shack, BMC, HTC Columbia, Liquigas and many others. I've done surprisingly well against them.
Do what the others said - borrow someone's old (or new) jersey or go naked (plain jersey). Don't race in another team's jersey if you can avoid it. Definitely don't show up in a pro team kit, unless it's 7-Eleven or some other 70's or 80's classic.
Best not to show up in one of those 'tuxedo' or Dr. Seuss jersey's either.