I'm having issues even getting together in the lobby with a friend. For him it says we're both in the same lobby, for me it doesn't, sometimes vice versa. Then when we finally get together it doesn't find the replacement players and have to back out and start the entire thing over again.
343 really needs to get their shit together with their matchmaking and their menus. It's a joke at this point.
If you're in a party, go to party>leave>leave multiplayer, it will break your party up. Also in the bottom right, you should see two different sets of numbers, the one on the left is your party count. If it says two, you will be matched with your teammate, even if your screen doesn't update to the search lobby.
I wrote this in the community thread the other day,
Some tips for H5 MM.
-checking your scoreboard in game and hitting certain buttons moves you from party to game chat.
-Always back out after a match, the B option is there for the first ten seconds or so. The load screen takes a few, but if someone leaves during intermission, it usually errors out eventually.
-If you get stuck in MM away from your party, go to party, leave, leave multiplayer, it'll boot you back to the home screen.
-If you get the "your party has changed" have the people in that party leave MP from the party menu to break the party up so you can reform.
I've not had much issues following this. Sucks that you have to do it, but the game is early, but it already works better than MCC, so I don't think it shares whatever problems that game has.