If you want to grant extra permissions to some of your registered users in MediaWiki, you have to login 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 hit the Edit user groups button.

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 check box. Check the groups where the user should be added and press the Save user groups button. After that the user will have the permissions of the selected groups.