They have a gas grill and a Blackstone flat griddle.In that case you may just be better off cooking them complete at the party. What cooker will you have to use? I prefer the front sear method, sear till I'm happy then put them on indirect heat, cold side of the grill then just cook to internal temp. It wont be a perfect edge to edge doneness like Sous Vide but it will be close and will be better quality steaks over cooking them twice.
At home I have ben doing salt, pepper the steak. Then put a sprig of Rosemary, sprig of Thyme and two small pats butter on top. Then in to vacuum bag, vacuum and seal, and in to the sous vide pot. Then sear on piping hot flat griddle.
My wife has a nice herb garden, so things like rosemary and thyme are picked fresh.