join has the type `m (m a) -> m a`. That's the thing that really shows off the monoidal structure. People normally implement monads in terms of bind, but you can easily define join in terms of bind for any Monad: `join ma = ma >>= id`. So really, as long as you have a lawful instance of Monad written with bind the existence of join is your proof.
I do not even know what a monoid or an endofuncor is. While I enjoy math, despite not being the best at it, I am confident I never made it this far in my studies. I looked at the Wikipedia definitions, and I am even more confused now.