Good point @colman although I have to say that if the need is for a ‘peer-to-peer’ relationship (with only a single field showing) then I wonder what the meaning of the grouping would be and whether it might be served by another solution.
See my comments here:
If the issue of showing more info for children is solved, then peer-to-peer can be achieved with the addition of a relation (many-to-one) to a ‘group’ type.
For the time being, you can link items to a group and then add a (read-only) lookup to show other peers in the same group.