I am using Multisite (which I know this plugin doesn't support) but using the s2Memeber plugin and everything works fine.
I use the normal WordPress registration page as opposed to wp-signup.php
One problem I'm having is if someone enters an incorrect field on the registration page (such as upper-case in username) the invitation code is expired when they try and correct the error. Any help would be appreciated.
Cheers for a great plugin ;)