"They are from the restaurant."
Translation:Mereka dari restoran.
On top of my head I can think of two function of the suffix "-nya"
To indicate possession, as in "restorannya" (meaning his/her/its restaurant).
To specify a particular object, as in "ini restorannya" (meaning this is THE restaurant)
"Mereka dari restorannya" doesn't sound right in this context, though. Because if you want to point out that they are from a particular restaurant, you would be better off using "Mereka dari restoran itu". Just omit the "nya" if you mean a restaurant in general