How to assign users to groups in MediaWiki?

If you want to grant extra permissions to some of your registered users in MediaWiki, log in with your admin user, click on the Special pages link at the left column and then follow the User rights management link. On the next page, enter the user name you want to modify in the text box and click on Edit user groups.

Under the text box, you can see a section with all user groups available in your MediaWiki installation. In front of each group, there is a checkbox. Check the groups where the user should be added and press on Save user groups. After that, the user will have the permissions of the selected groups.