Most Helpful Girl
Like the other commenters said, approaching them yourself is always an option. You don't even have to show that much interest; you can strike up a casual conversation and make it look like you're just looking for someone - anyone - to talk to.
If you're not into approaching, then just make YOURSELF more approachable. You don't want to sit around wondering why these people aren't coming up to you. Move around, chat with friends, show how open you are. Smile at EVERYONE, or at least smile with your eyes. If you just notice them staring at you without smiling, they won't feel welcome to come over and initiate a conversation. Body language is key. If you walked into a room and saw two guys - one of them sitting down, sipping a drink, eyes darting around the room and making eye contact with you several times, versus this other guy who's up and about, chatting briefly with everyone, displaying great versatility and good conversation skills, looks relaxed and easy-going, grins at you when you lock eyes - of course you're going to feel more inclined to speak to that one, because you know he's going to appreciate your company more than the other guy.