We just finished building https://clubsall.com, we implemented this, we call them clubs.
e.g. when you subscribe to "technology" club, you get content from multiple "technology" communities. You can also post a message and tag multiple clubs.
The main difference seems to be that we have pre-defined clubs that aggregate communities.
In your case, seems like every user can create their own groups. Honestly, intriguing idea.
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