I guess this sort of recursion won't work as a role. A role can not depend on one of the classes it is consumed by, it should be independent and only define functionality that can be consumed by various classes. Cleary, this separation is not possible here.

So, why a role at all? Are there really other classes that are very different from Group that can also cluster? If they are similar to the Group class then I think this would better be designed using inheritance, where Group implements the "cluster" method and then you may have sub-classes of Group (or come up with a new parent that Group is a child of too) which modify the behaviour of Group.