I'm not near a computer but a from what I remember you are looking for /execute [selector] x y z [command] So your command would be /execute @a{[team=Hider]Inventory:[{id:"minecraft:chainmail_boots"}]} ~ ~ ~ effect minecraft:resistance 1 3

I'm not near a computer but a from what I remember you are looking for /execute [selector] x y z [command] So your command would be:

/execute @a{[team=Hider]Inventory:[{id:"minecraft:chainmail_boots"}]} ~ ~ ~ effect minecraft:resistance 1 3`