I like the neat feature that ANYONE can join your game but if you set it to private then you should be able to invite your friends. I believe three people are needed in order to enjoy the game, if you have four though it tends to get a bit messy and like a friend told me the game can be quite competitive, just because the simple fact that there is very little ammo but if you have a Soldier Class there you can easily regain ammo with his special abilities.
If you are planning to play with a friend you better make sure you guys are at the same level if you have someone who is a higher level and you're playing their game then there is a huge chance you'll die faster and your weapons will not kill any monsters. But here is a tip if you decide to level up faster ask a friend who is a really high level to join their game so that way when they fight monsters your level goes up quicker so you can level up without killing any monsters. It's really easy and a much better way to complete the game. Though the game needs to be completed in order to do it by yourself.
All in all this game is decent but once you beat it, you better have a lot more money so you can purchase the DLC.