How do I play with a guest on garden warfare?

Sorry, that's not an option in this game...

If you are on Xbox One, you can play split-screen co-op in Garden Ops mode but there are no online guest options.

yeah as mentioned you can't play in the pvp (player vs player) enviorments, but you and your "guest"/split screen can play co-op garden ops mode. 

In order to play as local co-op you will need to be offline though.