Try this ASL (I'm not a fan of slashes in front of commands by the way...):
command </who> for each object in game if property <#quest.thing#; netplayer> then msg <#@quest.thing#>
That should just print a straight list of players. Obviously you can make it more complicated and add fancy formatting if you like.
For players to type some text to say to everybody in the room, just put a quote character in front of it. This is covered in the Quest help file under Playing Games > Multi-Player Games.