You can leave a faction mid-war and join another faction while they are in war. You can only do it once so they say. I've seen multiple leaders in one faction too.
Try this out as it works! Start a war with only one player in the faction. When your faction is paired with another faction let's say it's 1 vs 1, bring in a second account into your faction. The other faction will only see 1 player on your side. Next war declare with 2 players and when you're paired again, bring in a third player etc...