I met a girl at a club last night, shes seemed really into me for most of the night but my friend also liked her and towards the end of the night told her I have a girlfriend. (Which I don't). After that I couldn't find her in the club so didn't have a chance to get her number. Turns out my friend did some modelling work with her a few years back and has her on Facebook. I'm thinking of adding her but don't know how to do it without it seeming like I was stalking her over the internet! any tips?

First of all, just a couple of things:
Don't just add her out of the blue, it's real creepy. What you can do instead is send her a message and chat with her and eventually strive towards her adding you. (Not a must though), anyways you should wait some time before you do that and be honest and say that your friend told you he has her on Facebook, or better yet don't say how you find her, but, never lie because it'll be transparent as hell.

Another idea for you is to ask your friend to use the "suggest friends" thingy (if he's the same friend from the club). That way you don't seem like a creep and you're off from a fresh start.

Do that and go straight towards your text game.

Hope that helps bro!