yahoo said The ability to publish user-created chat rooms in the public Yahoo! Chat directory is currently unavailable. We are working on improvements to this service to enhance the user experience
Yahoo Ref ...
http://help.yahoo.com/help/us/chat/chat-03.html
you can still make a room but it wont show on the user list guess you would have to invite your friends to your room...
the /join roomname

command works if you know the name of the user room like if the room was called carol you would type /join carol
would take you to carols room ok no space between the /and join then a space and room name /join carol