code: private void primUpdateMembers(boolean renewLock) throws GroupsException {
          getLockableGroupService().updateGroupMembers(this, renewLock);
          clearPendingUpdates();
          this.invalidateInParentGroupsCache(Collections.singleton((IGroupMember) this));
      }
docs: Ask the service to update this group (in the store), update the back-pointers of the updated
  members, and force the retrieval of containing groups in case the memberships of THIS group
  have changed during the time the group has been locked.