You can do it a few different ways.. the first one that comes to mind is to have seperate instances of the video chat application on the server.
if you know what all of the rooms will be beforehand, you can just hard-code them into the client end. Put them in a listbox, and when selected, connect the user to that application instance on the server. With flashcom you can create multiple instances of an application. So if you have an app at "rtmp://myserver/video_app/" you can create unique instances of that app by connecting to "rtmp://myserver/video_app/room01" and so on.
If you don't know what they'll be called, you can have the server handle it. One idea would be to setup a single "lobby" application that the users first connect to. This app. on the server is connected to or at least knows the names of all of the created video chat instances and either stores their connection strings in a shared-object, or an array that it sends to each user when they connect to the lobby app. When the user selects a room, they get disconnected from the lobby, and connected to the instance they chose.
Hopefully that helps get you going. If you get stuck or don't know where to begin code-wise, let it be known! This would make a pretty useful tutorial thread.