To manage users you will first need to ensure you have set up a banding of privelidges. Admin & User should usually be enough unless you have any specific requirements. To set up the User Groups go to this section first.
If you are already logged into the administration area click onto the Manage users section
This will show you a list of the registrants.
Click the name of the person you want to manage
Check over their details and move onto the "Groups" tab when you are ready.
Tick which box this person applies to and that's it, done.