So my group of friends have been playing Borderlands 2 for PC/Mac (1 guy is on Mac another is on PC). We have been enjoying it a lot but we are close to finishing the game. I'm trying to find a good multiplayer game that can be played on both PC and Mac. Been searching through Steam but haven't found anything good. Wonder if Giantbomb community has some suggestions.
Check out http://store.steampowered.com/search/?os=mac&category1=998&category2=27 for more.
League of Legends would be a good pick especially since you already have most of team put together.
I'd second league of legends. It is so much better to learn to play the game as a group then jumping in solo. Expect to see ragers and afkers, yet if you are with a group it can still be great fun.
Awesomenauts (for 3 friends) and Guns of Icarus Online (up to 4 can man a ship) can be quite fun. They're both for PC and Mac, don't let the wiki confuse you (I played both with friends on different operating systems, while I played on Windows). Guns of Icarus Online can be a miss, though, since the community isn't that large for it, but that's probably because the game is a bit of a work in progress. Still, I've had fun whenever I played it. Either way, I think something else would be a better investment now, and wait for these games (if they look interesting to you) for when they go on sale.
I've heard dungeon defenders is fun but have not played it myself. Serious Sam 3 is a ton of with a friend and i can imagine it can get crazy with 4 players as the enemy numbers scale.