My guess is that you're probably copying and pasting the invitation code, and you probably are copying an extra space at the end.  Then when you paste the code, the extra space on the end is causing problems.

Let me know if that's not the problem.

I need to know the Paypal email address that you paid with.

Can you either post it here (I'll delete it after I read it), or send it to us at:   support [at] markettheme.com

I'll take care of it.