Manual:$wgGroupsAddToSelf

Details
$wgGroupsAddToSelf allows users to add themselves userrights.

The syntax since 1.14 is:

The syntax of MW 1.12 - 1.13 is:

Changes in 1.14.0
In r39368, the way this variable works was changed. Before, it allowed every user to add those groups to themselves, now it can be filtered on a per-group basis.

The new syntax is  which allows any user in the group "group1" to add the groups "group2" and "group3" to themselves. The old syntax will still work for allowing every user to add groups to themselves, but it is recommended that you use  instead.

You may use  to allow users in that group to add every group to themselves.