[kh99]

You can do this using a promotion. Go to Promotions under "Usergroups" in the adminCP and add a new promotion with strategy "Reputation".

Promotions are done by a scheduled task that runs once an hour and promots any users who have been active recently, so not all users who qualify will immediately appear in the new group. But if you want you can go to the Scheduled Task manager and press the "Run Now" button, and that should promote all users who qualify reagrdless of their last activity time.