You have to build it up: hang out with him/her with his/her friends and your friends. This will not make the person uncomfortable. When your next to the person, put your foot next to his/hers or you could put your shoulder next to them. If they pull away, don't try it again. The big question should be asked like this: you should ask him/her face to face-with your and their friends around. Say it casually, and give him/her a day to think it through.
