Can I use facetime to call between the USA and Europe?

Do you know the phone number or the email address of the person that you want to FaceTime with? If so, use that to contact them. They have to be set up to use FaceTime on their end or you will not be able to contact them.

Make sure that you are both connected to WiFi as well.

Yes you can. Have you Facetime them before? If so, there's no difference between calling Facetime users within the USA or around the world.