If you want to play the Operation missions with your friend, first you need to invite them to form a squad. After that, the squad leader can start any Operation mission in the co-op version by clicking on Squad CO-OP button. This feature is also connected with the Guide system.
What is the Guide system?
You can be a Guide for less experienced players, starting at rank 15. If you want to be a guide, you need to enable the option on your character profile. With the guide option enabled, occasionally, you can get an invitation to guide someone on their operation mission. After accepting the invitation, you will be transported into the Operation mission with a newbie. When the mission is completed, you get a temporary fame buff as a reward.
