Crowdstack doesn't have a concept of 'friends.' Instead, you can follow/get notifications when a specific person posts content. There's no mutual relationship. I'm not sure what that comment was referring to.
The reason our private messaging works the way it does is that it isn't really a chat, it's a thread, which means the content is persistent forever (until it's deleted). Every participant can see the history of the entire conversation. If you could add people to the conversation down the road, it exposes potential for comments that weren't intended to be seen by that person to be seen later. In our system, when you are posting in a PM, you know exactly who is seeing it.
For more ephemeral, ongoing, freeflowing conversations, we do have live chat rooms you could open and allow people to come and go. If you create a chatroom inside a subgroup, you can also restrict access by virtue of the subgroup access.