Manual:$wgGroupPermissions

Details
$wgGroupPermissions is a two-dimensional array indexed by user group and available permissions; for example, " " allows registered users to edit. These groups can be assigned to users through the wiki Special:Userrights interface. User group names can be no longer than 16 characters.

For in-depth documentation, see Manual:User rights.

Default values
Default values vary from version to version. You may find the one which apply to your mediawiki setup in DefaultSettings.php. For more info visit Manual:User rights.

Use by extensions
Some extensions, such as RenameUser or MakeBot, add new rights which can be configured and assigned in the same manner.