i have an invite, and a username, but the two wont play nice

hullo! please help! i recently signed up to be on your invite mailing list, only to receive an invite from a friend today. so now i have an invite code but it wont let me use my username or e-mail because it thinks it's already been taken. i'd really love to blend my invite code and my username into one neat little package. help! and thanks. :)