Stai al ristorante o sei al ristorante?
I'm sure only one of those is correct, but I'm not sure which. Grazie e prego :)
I think both could be correct, depending on the actual meaning intended. Under certain conditions, "essere" ("to be" -- the "sei" in your second option) and "stare" ("to stay" -- the "stai" in your first option) can be used somewhat interchangeably, but note a subtle difference: "stai al ristorante" -- you are at the restaurant and you are staying there. "sei al ristorante" -- you are at the restaurant, you may or may not remain there for any time.
"sei al ristorante?" translates to "are you at the restaurant?".
"stai al ristorante?" could be interpreted as a slang meaning the same as the previous question or could be translated to "will you stay at the restaurant?".
The first question is (in my opinion) the correct one.