Well, I’m still trying to wrap my head around the protocol, and my analysis also points to a relay specialization for running Concord groups. Concord introduces special features, such as the gift‑...
My two main takeaways were 1) the deletion thing, relays need to prevent it and 2) anyone can invite anyone, and its possible to lurk these invites until the next epoch (silent members can hide until they actually post or publish a join, but, if they do not post or publish they will be removed from the next epoch). Just an interesting behavior worth noticing. Could be used for example, for a push notify or something.