Guppe is the closest thing to a conference, though it's all public.
DMs with long member lists will go to everyone named. That's kludgey, but about the closest to a private list you'll find on the Fediverse presently.
If you want to get a room, you'll need to use a service offering that. Email lists, group forums, and the like.
(You should know the commercial offerings. I can't say I know any FS/federated options offhand.)
Guppe has been down.
As I mentioned to @jonny , I have been experimenting with the group service created by @piggo - see @organizingInFedi or https://git.ondrovo.com/MightyPork/group-actor#group-usage
Works something like Guppe, but has more capabilities, listed on that page.
Not private, not the same as a room.
How do you think a room might work in fedi? What could the UI/UX be like?
http://bonfirenetworks.org/extensions/ is working on some features that might help, like http://bonfirenetworks.org/extensions/boundaries.html
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