You don't, because MW3 isn't out yet.
You get invited into a private lobby and accept that invite that is how get into a hacked lobby.
Yes.
Yes, they can hack you from your account. Even if your offline, online, or in a hacked lobby they can still hack you
Yep. You can go to lobby leaderboard and look up your statistics.
press R1L1-CIRCLE LEFT LEFT UP UP RIGHT RGHT DOWN DOWN L1R1 IN A LOBBY
Yes, it has it, but as far as I've been able to tell, in MW3 the "Join" option only works if your friend is in a game lobby and there is an empty space for you. It doesn't work if your friend is not in a lobby, and they're just setting up their custom classes, or looking at stats, etc.
mw3 is part of mw3, it's call of duty mw3
mw3
mw3
There are no zombies in mw3.
mw3
There are 10 Prestiges in MW3.