I'm finding the single player really difficult once you get further in.  My question is will the difficulty remain the same with more people?  And I don't mean like its easier because theirs more people.  I mean will the enemies take more hits.  Stuff like that.

Yes and no... No because there aren't any more monsters to kill than there would be during single player. Yes because there are more spells flying around and allies spells can hurt you particularly careless Super Steam Beam aim, not to mention killing yourself by trying to cast Revive while wet. 

Enemies do have more health with more people in the game.

It can be but if you play with somebody who will cross beam with you on incoming enemies you can do some major damage that you can't do in singleplayer