In general I love taking pictures with people. For some people that seems to be a great memory for them to show people all of the cool costumes they saw at the convention. The easiest way to go about it is to ask permission first and listen to the person response. While most people are okay with being touched by a stranger, some people really aren't.
Simply put when posing with the some either stand next to them in a very casual pose (hands in pockets or on hips aren't bad ideas, even throwing up a peace sign might not be all that inappropriate, just do what feels natural). If you'd really like to get cozy with the person asking "do you mind if I put my hand around your shoulder/waist" is a good polite way to ask. If they say yes, you now have permission to do whatever you asked in a gentle manner. If they say no, smile, nod, accept it and pose with by yourself.
It doesn't really have to be awkward or weird. Just be polite and understanding and you'll likely have a lot of success~