This has been a persistent issue and seems to come in a few overlapping flavors -

1.  player A is locked out of any game which includes player B

2.  player A can only join games with player B, but locks everyone else out who tries to join after

3.  any two of player A, B, and C can join each other's games, but the third is locked out

4.  player A can join any game, but once connected to a game with player B locks out all subsequent players

Did I miss any?

You also see issues in games over 2 v 2 where about 30% of the time the 6th player to join will be unable to establish a connection to one or more players.  Once this happens that player will never be able to join that game, regardless of restarting client or retrying connect. 

The result of all of this is that as long as you are not looking to get into any specific game with any specific players you can get a 2 v 2 pretty quickly with 80% success.  Where the "80%" model breaks down is when you want to play with specific people or in a specific game.  Then I would drop the success rate down to about 20%.

Yes, and in addition to this a point might be:

5. Player B can join a game that Player A hosts.  Player B cannot join Player A's hosted game if any other player has yet joined.  Player B has not successfully connected to games hosted by any other players.  Player B has not successfully hosted any games (even with a single human opponent.)

There are other things which I think are obvious to people that have spent any time trying to coordinate games with specific people as well...

1. If someone who can't join tries, they effectively kill any chances for normally valid players to join.  E.g. Player A hosts, Player B can always join Player A's games.  However, Player C tries to join first, and fails...Now Player B cannot connect to the game, neither can players D, E, F.

2. If you want to have another shot at joining or hosting after any games are completed or after joining or failing to join a game, you must close and reopen the application.

It appears a lot of the trouble here has to do with successfully closing failed connections and connection attempts.  Maybe the newer netcode is doing this now but we are impatient in waiting for a timeout/error?  A wait of 60 seconds to join a simple game lobby is probably unacceptable for most users, especially without a progress indicator.

A lot of these could be at least mitigated if we offered more verbose indicators to the client.  A status which shows "Connecting to Yuriken, negotiating, Success!  Connecting to RedCheckers, negotiating, negotiating try 2/3, Failure!  You lose, reboot your machine, your router, your mom, and try again!"

I did want to update - Seriok was in a game and another user was able to join afterwards, allowing us a 3v3 against comps...I don't know what could have changed.