Roles and permissions

With Moodle 1.7 you can make users members of a Role in a Context. Moodle's role system doesn't allow assigning roles to groups (as far as I know).

Here's what you do:
Log in as an admin, go to homepage, 'turn editing on', click the assign roles icon (face + mirror). Now you'll see an Assign Roles screen. Click 'administrator' (since that's what you want), find your user and move him from 'potential users' to 'existing users'. Note that you're in a particular Context here, you're not assigning this user admin rights over the site.

Hope it helps!