1. Message them ALL on facebook (i.e. group chat), even the one or two people you barely talked to
2. Tell them you think they were cool people [and woo them some more with highlights of the last time you met them, compliment their performance at the cultural show etc] and finally tell them that you would like to hang out with them again.
3. Find a meeting place or event, nothing too long if it interrupts their schedule. (Meet at a pub?) invite them
4. Not all of them may attend cuz of their schedule, so just make a seperate plan for them.
5. Get their numbers (optional... add them on instagram.... i noticed more people are on instagram time-wise than facebook so take from that what you will)
6. Make it happen... meet up!
7. And goddamit, ASK them what they mean by their inside jokes! Keep asking them 'till they learn to stop being so fucking rude and aknowledge your non-insider existence.

It's not awkward in the least. It's probably more awkward for them to respond than for you to make the inquisition. But if they say yes (and who fucking cares if they say no), the Ben Franklin effect occurs and they'll grow a liking for you. Think of it like this, people are doing each other favours when they do the act of hang out with each other.

Also, don't assume your roomate is an asshole unless he literally says "I don't want you to be friends with my friends". It's usually their own personal problem or maybe they are socially challenged, yet they don't mean to intentionally harm you.

I've done this before and pretty recently! It may be challenging but definitely not awkward.