But ... after 20 seconds of thinking you probably could do this nicely with the User Defined Table Module. Use of this module is covered quite nicely here:
User Defined Table (UDT) DotNetNuke Core Module.I would create a list that has these columns: Name, In, Out
In and Out would be true/false and be represented by radio buttons. Or just make one Status column.
You could give everyone the ability to edit the module, so they could change their In/Out status.
If, instead, you wanted to create a simple module for this, that would be a fairly easy thing to do.
If you want to create a module and don't have the resources or know how to do it, I recently discovered this site - www.freedotnetnuke.com. The offer to create modules for free. I have no idea who they are or what their game is, but if might be worth a shot. (I would, though, probably want to see the source code before installing it on my site!)