You could possibly already @-mention other members within the group based on their key fingerprint or any unique prefix of it, just like it is done in git. No further data would need to be maintained and synchronized then.
However, I still believe that user ping mentions would be more durable if it was serialized as an openpgp4fpr or i.delta.chat URI (only with the fingerprint and address included) in the sent message. The Delta Chat interface would render such links as a clickable user pill with the username and avatar for the given user in the group.