Even if we suppose that she has no boyfriend, these aren’t signs that definitively show that she is interested.
She has only stared at you a couple of times. That is too little.
She could have easily been daydreaming when she was staring at you and thinking about something completely different.
The smiles could merely indicate that she is a polite person and smiles at everyone that greets her.
Instead of adding her on social media at this time, try talking to her first.
Break the ice with her and establish some initial rapport.
If the conversation goes well, you could both exchange social media information at that time.