Bumping this to let it be known:
It's not just +__+ who has the issue.
Any user with a + in their username cannot be added. (I had trouble adding the account Cane + Able to a hydra PT.)
This seems to be a rather serious problem, because, uh, yeah. In ANY game that uses private chat, you have a compromised slot because you know the account with the + cannot be mafia or at the very least, can't communicate with them.