Manual:Setting user groups in MediaWiki
By default MediaWiki has several user groups that can grant a user a set of user rights. See also the list of groups.
Special:UserRights (MediaWiki 1.10 and higher)
|MediaWiki version:||≥ 1.10|
There is a simple interface (Special:UserRights) for adding a user to or removing a user from one or more user groups.
To do this the acting user must have the
userrights user right, which by default is granted to the "Bureaucrat" user group.
You can access Special:UserRights directly or via Special:SpecialPages in the toolbox on the left (as of MediaWiki 1.16.0).
From source code
As documented into the https://doc.wikimedia.org/mediawiki-core/master/php/User_8php_source.html page.