So that users can see who might see what they're saying, the client should display a roster of users currently in the room (i.e. any the server considers active).
The API will therefore need an action to list active users, and the client will need a new pane (which unfortunately means getting hands dirty with urwid).
Will need to put some thought into how often it's refreshed. Although it could just be updated when a new user joins, that'd mean there's a remote possibility of a user not having a fresh list if they didn't receive the message (because their connectivity dropped for long enough for the message to be purged from the queue: if Bob joins, 10 seconds later Alice joins, then Carol only needs to have lost connectivity for ~21 seconds to miss the notification of Bob joining).
Conversely, don't necessarily want to send the list with every pollMsg response.
Timing is definitely something to figure out, the rest should be relatively simple to implement.