Played several games last night post update and I till get placed on the opposite team from my guest. Does the same thing if a friend logs into his gamertag on my xbox. We have only been on the same team once since the game came out.
You will have to stay patient and wait till they fix mp.
I was also playing last night for about 3 to 5 games with a party of 2 or 3 and most of the time we ended up on different teams. I also played with a guest the other night and had a little better odds with being on the same team. I have the same problem as you and I am sure they are tracking on it and fixing it. Kind of a hit or miss for me.
It sucks but it shall get fix in time.