I'm pretty sure it has to do with a combination of all the stats + bonuses of all the club members -- including the base str/intel, tutor bonus, RS bonus, misc bonus, etc. Contrary to popular belief, it does not appear as though successful party completion has anything to do with it (i.e. forfitting a party will not hurt your club rank). Therefore, clubs geared to high stat players and big spenders will almost always be at the top of the leaderboard. This is why you see a lot of premium party based clubs up there, because those attract spenders.
I've been told it's total main stats only but what Muschi says above sounds about right. I would like to point out 2 of the top 3 are potd and 6 of the top 12 are potd 🥳