Once I went and sat right next to her and the other I asked her to come sit next to me (which she refused). So I'd like to know what is the most efficient tactic to follow when coming up with this kind of situation?
Asking a girl to move is generally an early compliance generator. What's going to work best here is persistence in the instance of her refusal. Convince her to sit next to you through multiple attempts. Here's how this may play out,
You: Come sit next to me.
Her: No thanks, I'm fine over here.
You: Sitting next to each other feels naturally better *gesture towards spot you want her to sit*
You can generally carry on with things of this nature after her disapproval 2-3 more times before letting it go and allowing her to sit there. I've found that the best course of action is to only ever tell her to do it one time, and then progress into convincing/leading.
Telling her to do it multiple times announces that you're trying to get her to submit, which by nature shows extra effort towards making her submit, which in itself shows a loss of power and frame on your side, making her less likely to submit. Very circular logic, but it pans out in reality.