Right now clans are leveling up by recruiting players with Founder Status. Founders give the first clan they join 500 XP. Soon Elite will have Clan Challenges and Clan Operations which will rank your clan up. But that's not working right now so...
Ive been trying to figure that out as well. So far I have found out that the clan challenges are mostly maxing out stats on guns etc. I doubt if we will get clan challenge xp until elite is finally working. I just hope that the challenges we have finished will count when it finally does work.