This guide will tell you how to LEAVE PARTY in Call of Duty Modern Warfare 2.
First, you have to boot up your game and then press the r3 button to open the social menu. After that, press X on the bottom left side where it says your party.
The new window will open on the right side of your screen. You simply need to press the triangle button and then leave the party. Just select yes and then you will be out of the party.
Now the second method to get out of the party is by going back to the home screen and then clicking on the lobby. After that, select Leave Party and then the same notice will appear. You have to click on yes and you will be out of the party.