The Phabricator project offers a great communication channel between users and upstream maintainers. Seriously, this is something very hard to find in big, complex, mature open source projects like this one.
However, when it comes to communication between Phabricator users... There seems to be nothing? Unless there is an active channel somewhere that we have missed.
The use cases of a Phabricator user group would be the typical: basic support, professional services, meetups... How big and how far always depends on the energy of the group. It should be self-sustainable, without requiring much extra work from the maintainers. In fact, it should save them work, filling gaps that they can't or don't want to fill.
Maybe it is just a matter to have an own project here, and use Maniphest to handle the communication?