You just need to add the person in question as a friend on your Call of Duty account. You can then invite them to join you, or you can join their party directly. This is all easy to do in-game, with no third-party tools required.. Mind you, the farther away they are, the worse your ping will be. A gaming VPN would still be a good idea for both the extra security, and because it can actually reduce your ping under the right circumstances.