Manual:$wgGroupInheritsPermissions

Details
Groups that should inherit permissions from another group.

This allows defining a group that inherits its permissions from another group without having to copy all the permission grants over. For example, if you wanted a manual "confirmed" group that had the same permissions as "autoconfirmed":

Recursive inheritance is currently not supported. In the above example, confirmed will only gain the permissions explicitly granted (or revoked) from autoconfirmed, not any permissions that autoconfirmed might inherit.

Note only $wgGroupPermissions and $wgRevokePermissions will be inherited, not $wgAddGroups, $wgRemoveGroups, $wgGroupsAddToSelf or $wgGroupsRemoveFromSelf.