Enable moderator approval of registration from CB > Configuration > Registration. Once done their registration will need approval. You can then edit them from backend, change their usergroup, approve their registration and they can then login (they'll get a welcome email, if configured of course). Now have your various plans set to only be accessible to your various usergroups (see Access tab). Once they login they need only to review their Subscription tab and select a plan to subscribe to (they'll only see the plans relevant to their usergroup).