MediaWiki:Userrights-groups-help

You may alter the groups this user is in: $wgWhitelistRead
 * A checked box means the user is in that group.
 * An unchecked box means the user is not in that group.
 * A * indicates that you cannot remove the group once you have added it, or vice versa.
 * More information on what each group means can be found at Help:User access levels

$wgGroupPermissions['*']['edit'] = false;

$wgGroupPermissions['user']['edit'] = false;

$wgGroupPermissions['sysop']['edit'] = true;