how does it work? do i need to put these ninjas on team in order for the attribute bonuses to work?

or i dont have to put them on team, and simply just have to be "water main"to avail of bonus from Mei Terumi, and ëarth main"to avail of bonus from Onoki?

You equip boosting ninja just like a summon. It affects ninjas of a certain nature, for example temari buffing wind attribute attack can buff minato (wind nature) but wont buff sasuke (lightning nature). Total attribute bonus (not % one) is added to your whole lineup.

For some reason if i switch from Mei to Danzo it doesn't affect my BP...which is weird since i only have 1 water ninja in the lineup and 3 Fire ninja so Danzo should give me more BP.

Any opinion on that? is it a bug?

It is not a bug, the switching of ninja doesn't give any power since it's a "buff" which is used in fight

the power gain is only from upgrading the "maps" of your cards in the feature

so the nin, resis, attack, def bonuses given by each ninja doesn't add all up?

the only thing that adds up in total are the "maps" attributes that looks like myoboku stat bonuses?

hope someone can clarify... or Ice?

You are correct. Only the myoboku-like part adds up.

% bonuses work only with the selected assist.

The % part works like the scrolls in treasure tools.

You basically have 1 scroll that say since round 1 onwards your water characters have +2% nin +3% res.

Exactly ^