Alice, Bob e Carl are in the space titled "ABC Space" and is used by them to pass on information.
David is not in this space but he wants to pass information to all three.
So, when a space is created, make it possible for this space to be visible to all accounts in the organization and then give David the ability to view "ABC Space" in the search in order to send messages.
David, in his account, will not see the messagge history of this space (because he does not belong to this space)
One of the three members of the group, Alice or Bob or Carl, can answer David in the space "ABC Space" with a mention for David.
Alice responds in space and her message is seen by Bob and Carl.
David sees Alice's response in the her "under space" "ABC Space"
Can you provide more info on the use case you are trying to address?