Number of channels a peer is in?

Competitive puzzle game template --

FRAME 1 - name entry screen - connecting to "chat" channel
FRAME 2 - chatroom/challenge lobby - click a person's name, challenge them to a match if they're not sending/receiving a request already - connecting to "[peer_name]" channel
FRAME 3 - test room to make sure both players are joined in the same "[peer_name]" channel, as a precaution
FRAME 4 - the game itself

The challenge lobby has a List object that loops through all the people in the "chat" channel - the player clicks a name to highlight it, then a Button to send a match request.

Questions --

1) If a peer is in two channels (the "chat" channel and one named after a challenged player), they will be in the fourth frame and unable to bounce messages back to you. Is there a way to get the number of channels a peer is in, and use that as the condition for an event that tells you "this player is in the middle of a match and cannot be challenged"? Simply disconnecting in-game players from the "chat" channel (and by extension erasing their names from the List object) may give the impression that there are no players available in the lobby at all, rather than players simply being preoccupied.
2) If a peer is in two channels, is there a way to format their name in the List object differently from the other lines in the object? Say that active peoples' names are formatted as normal, but in-game players' names are italicized and greyed out?