Github offered organization accounts to teams last season (and probably still do?).
Here's the topic from earlier this year:
http://www.chiefdelphi.com/forums/sh...ghlight=github
Note: I think you can have an organization account that is free but all the repositories are public. This isn't normally a problem in FRC.
They even have a FIRST octocat!
http://octodex.github.com/FIRSTocat/