@z428 yes everybody has to be in that forum or group. That what you are trying is something like a "private public post Chimera".
You can do that if you have every other node in the fediverse under your personal control. Since this might a little bit difficult and guides to some serious discussion potential besides the node owners (and users) you should maybe rethink the reasons why you want to do that.
The more polite way of "grouping" people is to ask/invite them to join a group/forum by their own free will. There might be a chance that some of the people are not happy to be put in a half-public "Opt Out" selection by some strangers. :smiley187: