Manual:$wgSpecialPageGroups

If the only parameter is a string, that is the page name. Otherwise, it is an array. The format is one of:
 * array( 'SpecialPage', name, right )
 * array( 'IncludableSpecialPage', name, right, listed? )
 * array( 'UnlistedSpecialPage', name, right )
 * array( 'SpecialRedirectToSpecial', name, page to redirect to, special page param, ... )