The Steam edition of the game supports drop-in/drop-out co-op multiplayer (via SteamWorks), so you can play with people who have the Windows version of the game. The Mac App Store edition will only allow co-op with players of that edition (see this article on setting up multiplayer with the Mac App Store edition).
The game has been designed with co-op in mind, and as with Diablo III, more players means tougher enemies but better loot. That said, you’ll probably want to keep a character for each group you play with, as everyone in a group needs to be at a reasonably similar level in order for it to feel properly balanced. As mentioned earlier, some character classes seem more appropriate for cooperative play than others, but that will depend on your particular play style.
Several expansions have been released that add new locations, missions and playable characters to the game, as well as those that increase the level cap. A “season pass” is available for the Steam edition of the game that includes most of it, there’s also a “Game of the Year Edition” of the game that comprises most of the DLC.
There’s also an official map app for iOS devices available:
Aspyr have announced that they plan to make all the DLC available for the Mac version, but there may be a short delay (of a couple of weeks) between the Windows and Mac versions.