i guess if you introduce your coursemate to your flatmates would be a start, then if they get on, you kind of have a group, and maybe arrange a nite out and ask them to bring all their friends along and u may make friends with their friends too....
i doubt my advice is very good t though as i feel lonely too, i have made a group of friends, but i just feel like i dont have anything in common with them, and i sometimes get the feeling they dont like me that much! and i dont really know anyone except for them...so i dont know how to meet new people as everyones friendship groups seemed to have settled down now! any advice of getting out of this rut i feel stuck in?!?!