Is there any way to use the admin feature to add an already registered user to a new group? I need to have one of my users become administrator, and I can't see any obvious way to do it except by editing the table directly in acl.auth.php...
Go to the "Admin" section, then "User Manager". You should see a list of the users on your wiki, if he isn't displayed - enter his username in the filter to find him. To edit details for a user, click the little icon with a pen in front of a man between the checkbox and username. On the lower right you will now see fields to edit the details for that user, including a list of groups the user is in seperated by commas. Write your administrator group there.