For a simple dinner party. I am serving roasted root veggies and salad with the steak.
I personally find that Excellent meat benefits from very little. Pepper, Salt, and Garlic are my staples. Occasionally I'll do a marinade or a rub of some sort - but I like the flavor of the beautiful beef to be center stage.
I only used good sea salt. When using a not so great quality piece of meat I would marinate it in a little soy sauce, brown sugar, rice vinegar and garlic. Good luck!
I do like to let the flavor of the steak shine through when it is a good cut but I also find that a little seasoning helps. In addition to kosher salt and black pepper, I very lightly dust steaks with garlic and onion powders. Then, let the steak sit at room temp for about an hour.
Salt and pepper. Serve with lemon wedges. That's it. It's kind of a Tuscan thing. Hopefully you are grilling this "very good quality" steak over a real wood fire and not some wimpy gas apparatus.
I agree that S & P is enough, but if you want a rub I like a mix of S & P, a little sugar, garlic, chili flakes, porcini powder, and a little EVOO.
I think that skirt, flap, flank, tri-tip are enormously flavored with a little help with wosrtchire, soy sauce, Dijon mustard, etc. - some type of marinade besides salt and pepper. This type of meat is fine on its own but it takes on so much flavor easily.