Hi again!
I received some new input about what community users expect of this feature.
In a community there is a set of administrators. They collect groups into the community. Once inside, group members only can see their own group conversations, thay cannot see the chats of the other groups.
On the other hand, as community members, they can see the whole structure of the community, which groups are insde the community and which participants are in each group, and then, community members can ask to the members of other groups in the community to be added in those other groups.