"Teşekkürler, güle güle!"
Güle güle is said to someone leaving BY THE PERSON STAYING. It means "smilingly/laughingly/happily" and is short for "Güle güle gidin" -- so it's like saying "Go forth from here happily!"
Hoşça kal(ın) is maybe slightly more often said by the person who leaves (not a hard rule, though--can be said on the phone, or when two people part, or even by the person staying). It means "Stay nicely/well."
Generally not. Unless, you know, one person is dropping the other person off and is in some sense "staying behind" the other one. I could picture saying it to someone getting out of my car, or maybe if I walked them to the metro to see them off (but it's a little weird and nonstandard).