***** NOT A TEACHER *****
Good morning, Battery.
(1) You have asked a good question.
(2) If you are at home, and your wife asks about the location of your friends, you would say:
Oh, they are AT the restaurant. We will go there later.
(3) But if you are standing outside the restaurant by yourself, and someone asks: Hey! Where are your friends? you would say:
Oh, they are already IN the restaurant. When we arrived, they entered immediately, but I decided to stay outside to get some fresh air.
***** Have a nice day!