I'm having the same problem. When I first log in and start playing, I'm able to join friends and play a few war games. Then I get kicked from a game and from that point on I can't join anybody. It say's they are joinable, but as soon as I click the button, it says not joinable.
If I back out of Halo 4 and come back in, I am able to join my friends again for a few games before I get kicked again.