provide an url of registration form in your invitation..so they can register there..simply disable register page on your site...
or
in that invitation send them with some code of random number which is stored in your data base table...check with it,while user try to register in your website...
If your requirement is other than my post..let me know...
Shanti C
Posting Virtuoso
1,642 posts since Jul 2008
Reputation Points: 137
Solved Threads: 162