Are you talking about rrd4j.cfg ?
rrd4j doesn’t know anything about dealing with openHAB Groups.
That’s just an Item with a single value, the (Group) state.
The * syntax meaning “all members” is for the OH persistence service, as configured in rrd4j.persist, and rrrd4j knows nothing about that. What really happens is that OH figures out the members and persists individually to rrd4j
Can’t. Closest is the individual list method you have already found.
It would actually be pretty difficult. What happens when Group membership changes? What happens when an Item belongs to more than one Group?
I don’t know if any wildcard is available, so you can configure for individual Items with similar names as a workaround.