Thank you for reaching out to us.
The Usergroup is determined by the number of your Credits in the Forums. You can gain Credits and Prestige every time you post. You will need at least 50 Credits to upgrade your Usergroup from Newbie to Register. You can check your current Credits in your User CP.
You can check out the other requirements for other Usergroups in the User CP> User Group Page, here is a link for your convenience: http://forums.game321.com/memcp.php?action=usergroups.
We hope this answers your inquiry, should you have further concern, please don't hesitate to contact us.