Create one group called "Pre-Registered Users (after this called UG1)" and another group "Registered Users (after this called UG2)" (you may use the normal registered users group).
All right, now setup UG1 to access only one forum with one thread to informations about your subscription and link to
www.example.com/payments.php
Now when your users with UG1 pay, the payments move the user to UG2 and go
Sorry for my english..