Simples Assim

1) Log in to the database

2) Run the following statement:

INSERT INTO user_groups (ug_user, ug_group) VALUES (<User ID>, 'sysop')

http://www.mediawiki.org/wiki/Manual:Database_layout

http://www.mediawiki.org/wiki/Manual:User_groups_table

http://fernandoribeiro.eti.br/2009/11/27/mediawiki-extension-for-ldap-authentication/

You can also link to groups in the directory, as explained here.