The function to use is GetLobbyInfoArray like this:
- <div id="channellistwithusers" style="margin: 6px">
Channel List<br />
<%LobbyInfo[] lobbies=ChatApi.GetLobbyInfoArray();%>
<%foreach(LobbyInfo lobby in lobbies)%>
<%{
string sOnlineChater = lobby.JoinClient(",");
%>
Lobby-ID:
<%=lobby.LobbyId%>,
Title:<%=lobby.Name%>,
Users:
<%=sOnlineChater%>,
numOnline:<%=lobby.ClientCount.ToString()%><br />
<%}%>
</div>
But you need to poll it to keep it up to date. Would be nice to have Javascript update it. In SupportClient.js it looks like SUI_HandleChatEvent("USER" ...) does just this but I haven't tried it.