Tonight I met up with an old friend with whom I used to play a lot of Reach with. He was in a fireteam with another friend, and when I joined their party all seemed normal. But when the party leader started the match I got stuck in the loading screen. They were shorthanded for the entire match. This happened twice. Afterward my friend took over as party leader and I was able to join the match without an issue.
This leads me to believe you can only join games in a party that is lead by someone on your friends list.
Has anyone else encountered anything like this?