Hi Kai!
I second what Matthew said. In the case where you met the person recently, and you are not actually friends yet, you would say "I just met her."
However, there is a different answer if you are talking about someone you used to know in the past. If somebody asks you "Was she your friend?", and you would like to say that you were not friends, but you knew who she was, you could say "No, I just knew her." or "No, we were just acquaintances."
Please let me know if this makes sense or if you have any questions!
Savannah