OK, I just got an official response:
Hi,
Thank you for contacting PokerRoom support and please accept our apologies for the delay in replying.
In order to accommodate the large number of players recently, we have set up yet another play-money server in addition to our existing play-money and real-money servers.
Play-money players are located on one of the two play-money servers and real money players are on the real-money server.
Since these three servers do not currently communicate with one another, players of differing play status may see a different list of tables and be unable to find each other. If you and your friends are a group of play-money and real-money players, the only solution would be for play-money players to make a small deposit so that they can get on to the real-money servers to find their friends or watch the games there.
It is sometimes also possible that play-money players cannot find their friends, as play-money players are now randomly put on one of our two different play-money servers. In the meantime, if you are all play-money players, to find your friend, all you need to do is reload the lobby page, or restart the game application if you are using the downloaded software. You will then have a 50% chance of finding the correct server. Please accept our apologies for the inconvenience while our technical team is working on a better solution
Regards,
Andrew
PokerRoom.com Support Team