Smiles do not always indicate that a woman is interested in having a drink with, or even saying more than two words to a man. If a woman smiles at a man while holding eye contact for several seconds, it likely means that she is at least interested in saying hello and chatting for a few minutes. A simple smile, no matter how nice, without any sustaining eye contact is almost definitely just a smile.
There are certain cues that a woman gives when she is interested in talking with someone. Looking a person up and down, also known as "checking them out," is a big sign when coupled with a smile. Folding the arms and crossing the legs away from the individual are two signs that there is no real interest there, but keeping an open posture and touching her hair are good signs of interest.
If a woman smiles at a man repeatedly, it is almost guaranteed that she wants him to approach her. It could just be coincidence, but not likely.