Networking at work?

Im finding it hard to network in work. Ive been in my department which employs 500 people and two girls who started last october know more people than me and ive been here 2 and a half years. I am shy but id like to think im a friendly person even though i might not look the most approachable person cause of my shyness. Theres this one girl who has a habbit of seeing someone out in town from work then the next thing theyre best buddies and im like whats wrong with me that i cant make friends like that. I understand the girl is under the impression everyone for her own and wants to know as many people as poss. I just feel left out that theres this big group of people who know each other. Work nights are arranged in my sisters department but im not the most confident and worry people wont like me