With monads, can join be defined in terms of bind?

It is possible:

join :: Monad m => m (m a) -> m a
join m = (m >>= id)

Note the tricky instantiation of >>=:

(>>=) :: m b -> (b -> m c) -> m c
-- choosing b ~ m a , c ~ a
(>>=) :: m (m a) -> (m a -> m a) -> m a

so we can correctly choose id for the second argument.

Yes it's fairly simple:

join m = m >>= id