I was wondering, will matches be 4vs4, 5vs5, 8vs8 or will there be more players per game?

8 vs 8 in normal matches, in a custom game you can make it so that its 6vs6, 4vs4 etc :3

Thanks.

Also, will it have mid-game joining? I play a lot of Halo Reach and it doesn't have it, which sometimes results in unfair 6 vs 8 games -_-

if i remeber correctly yes

which is very cool

Full drop-in-drop-out.

So yes. Any time.

Not that teams will ever be unbalanced anyway, bots fill in for any missing players.