Oops I started writing a reply for this hours ago and got distracted.
Well I’ll use my privilege to post it anyway.
The Discourse software that the forum runs on does have this functionality, via groups, MrEyeball has set these up for Devs and for Mods: https://www.realmeye.com/forum/groups/
However I’m guessing there wouldn’t be willingness on MrEyeball’s part to set them up for things such as Minecraft, because there wouldn’t be much to gain by having a gated community for other interests within the already pretty small community we have on the forum, for things that are not Realm-related, when such convos can be had in #off-topic, albeit with no control over who reads/replies.
We do have access to group private messages via the main RealmEye site messages (your page). It was created before the forums, so it’s a lot more basic, but it works fine if you know in advance who you want to see the message. Less good if you want it to be a ‘living’ group message with adding and removing participants. The only option is to start a new message with different participants.
If truly desperate/dedicated, players could form a guild in-game since that would allow everyone access to the RealmEye guild message board, and control would be via the founder with promoting/kicking out of the guild, but, aside from being a very clumsy solution, as that’s not fully integrated with RE forum either, I doubt it’s the answer you’d reach for, as the same thing could be achieved way way simpler via a Discord server.
PM me if you wish this to be reopened for any more replies, otherwise you can just let this late reply stand on its own. Sorry for being slow!