So apparently I can tag along and play missions with someone I have not even unlocked in my profile (for example a 3/6 or 4/6 mission for an area) if I join that someone's lobby and he has to play that mission.
In the same manner the question I wanted to ask is that if I play in my friend's lobby who has yet to play a mission I have already completed, will playing that mission AGAIN grant me that mission's XP again or even half of quarter of the total XP or nothing?

Thanks to anyone helping