actually the party chat stuff is only semi right.. you dont need an invite, but the one leading the party has to be on the play online screen and not in a gamelobby yet.
The player that wanna join the party can use the partychat to get in. Go to the name of the host, or any1 who is in the party press options and then join game session.
This is the fastest and easiest way to join.
If you are in a gamelobby (the screen where you can vote maps) and youre friend tries to join, then he isnt in the party and the game will just random put him on a team