Please create a method to list all users in a group or that are connected to the service.

The API would benefit greatly from a call to get a list of users and connections connected to a group or hub.
When a client first joins a group, they may want to know who is connected.
Also I may want to poll a particular group to see who is online at that time.
This is more robust since it's tracked inside PubSub which already is maintaining such lists internally.

There is already a UserExists method so this would be a good comliment.
I propose you add a method such
- listUsers(optional group name)
- listConnections(optional group name)
- listUserGroups(userID)

Start by reviewing the existing UserExists method and the PubSub group or hub APIs. Compare the three proposed operations—listUsers, listConnections, and listUserGroups—and determine the required API behavior for group membership and online connections. Done means the requested listing methods are implemented with agreed semantics and covered by the project’s relevant tests.

Written by the indexing model from the issue text.