Representing Group membership in HA

[slyglif]

One of the common things I need to do when automating some lights or switches is setting a common custom cluster attribute to a single value, or executing a custom cluster command across all the members of a group. While setting the attribute will eventually be taken care of when herdsman #1318 is done, executing commands, at least on my Inovelli switches, would still require knowing the group membership and issuing the commands to the individual switches.

The main difficulty with executing commands from HA across all members of a z2m group is keeping the group membership in sync between the 2 systems. Could the relationship instead be passed / updated from z2m to HA? For example, in the mqtt discovery messages it's possible to list connections. Could listing the individual device's IDs as connections accomplish this? An HA automation could then iterate over {{ device_attr(device_id, 'connections') }} to execute the given actions.