Assume there is a class of a hundred students. Naturally, they won't all be friends - or, at least, good friends - with each other and all hang out together; they will form small groups of, say, five to ten persons.
However, there is always that one person (X) who is able to be liked by most people and who also tends to not have any problem with anyone. So, he/she has (a) friend(s) in each of the groups. Those friends would serve as the "points of contact" for the groups. So, if, for instance, X wants to invite the whole class to his/her birthday party, he/she can just invite the points of contact and tell them to tell the rest of their respective bunches to come too.
So, basically, X is at or around the centre of the network in his/her class. He/she might not necessarily be the only one, there might be a few others with that same characteristic.
What is the technical and/or non-technical term for a person like X?
P.S. By "social networks", I'm obviously not referring to online social network services (Facebook, Twitter, etc.).