Ask for the seat

Example 1

You : Excuse me. Is the seat taken?
A : I don't think so?
You : Can I site here?
A : It's ok for me.

Example 2

You : Excuse me. Does anyone sit here?
A : No go ahead.

Example 3

You : Excuse me. Is there anyone sitting here?
A : Oh. Yes. My friend is sitting here.
You : It's ok.