Most Helpful Girl
Approach them and ask them whether they are interested in hanging out.0
Most Helpful Guy
Women are almost always way ahead of guys on this. If she likes you she has already thought out a dozen scenarios in her head. All you have to do is fall into one.
Having said that, the other posts are right, just because a girl smiles at you that isn't an invitation. Smile back and say something and look for more definite signs of interest. Does she look at you intently, how close is she standing, does she touch your arm or shoulder etc & etc?0