That would make a lot of sense, especially since the EVENT_GROUP_MEMBER_LEFT already got the memberDisplayName added a while ago.
In the meantime you can use EVENT_UNIT_CREATED and ZO_Group_IsGroupUnitTag like they do in
zo_grouplist_manager. Instead of filtering it in Lua you could also use an
event filter REGISTER_FILTER_UNIT_TAG_PREFIX with value "group".