Mount your friends, Battleblock Theater, maybe something like Awesomenauts?
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Bomberman, any version that support local multiplayer. Bomberman 93 is a staple on any gaming nights with my friends.
Dungeons & Dragons: Chronicles of Mystara is great if you like beat 'em ups. One of the best that I've played at least.
Thief Town is supposedly rather fun as well. I have a spare key for it, so just send me a message on steam and I'll give it to you.
Worms, any of the 2D ones are good.
Jamestown, and it also supports a wide range of input devices, so even if you don't have a lot of controllers it's easy to play.
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Hidden in Plain Sight. http://store.steampowered.com/app/303590/
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Lovers in a dangerous spacetime might be smth for you (no Mac but splitscreen coop)
Edit:
Monaco seems to have couch-coop too (though I havn't tried it that way)
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Wyv and Keep. It's two-player coop, but the rest will have a blast mocking the players when they inadvertently kill each other. And maybe then taking turns to kill them instead.
Also, Magicka?
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Towerfall Ascension is a godlike game with 2player coop multiplayer or 2-4 player versus.
Broforce
Spelunky works well, but it's harder than fun - but multiplayer makes it easier with caskets to "revive" your friends
Hidden in Plain sight
Crawl
Sorry if any of these are not available for mac
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