@freemo Could this be achieved by a bot from within Mastodon? I'm thinking something like ChanServ on IRC networks, crossed with mailing-list forwarding.
You'd send, for instance:
(a)groups CREATE Moderators
(a)groups ADD Moderators (a)arteteco (a)design_RG (a)mngrif (a)surasanji
Each step can give you a confirmation that the group name wasn't in use, that the bot could find the named user to add, that you had the right permissions, etc.
Now you send:
(a)groups TO Moderators Hi, fellow mods!
The bot messages all group members, copying the privacy settings of your TO toot:
(a)arteteco (a)design_RG (a)mngrif (a)surasanji
(a)freemo said to Moderators,
Hi, fellow mods!
Later on you could send:
(a)groups REMOVE Moderators (a)surasanji
if you don't want that user to be part of the group anymore.
The bot might also implement commands like MUTE/UNMUTE
to restrict certain members from using TO,
PRIVILEGE/UNPRIVILEGE
to invest other members with op status,
OPEN/CLOSE
to enable self-service enrolment,
LEAVE/JOIN
to use self-service enrolment in an open group.
I've used (a) to avoid tagging and annoying my example users; it's meant to represent @ though.