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.