I have a 1.5 lb. tenderloin for 5 people.
How about making Beef Wellington?
My preference is seared on the stove-top and then finished in the oven. However, if you asked my aunt she would say grilled.
Often for a large dinner party one would sear the one large whole or piece of a tenderloin and then roast it in a 425 degree oven until it's medium rare. Then slice it into 1/4 to 1/2 inch slices and perhaps offer a sauce or choice of sauces...bearnaise, horseradish and a brown sauce (madeira, mushroom) of some kind would be classics. Since your party is small you might want to offer everyone a chance to have their steak cooked how they like it.
In this case I would cut the tenderloin into 5 equal pieces and grill them to order and serve with a flavored butter or a sauce
My own opinion is that the best way to cook any steak is outside over a lump charcoal fire. Cook to an internal temp of no more than 130F. If you only have a gas grill than you might as well cook it in the oven. Gas doesn't improve anything. Sauce choice is up to you. Because the tenderloin doesn't have much marbeling (fat) you have to keep a close eye on it; meaning that it's not going to be basting itself from the inside. When you are satisfied with doneness be sure to let it rest for 10 to 15 minutes tented with foil.
Season liberally, place in a Foodsaver bag and vacuum pack. Place in a pot with hot tap water at about 130*, using a probe thermometer. Add hot water or ice as needed to maintain temp, you can do this in a small Igloo cooler for better heat retention. After a couple of hours, (while you prepare the remainder of your dinner at a leisurely pace) remove from the bag and sear in a screaming hot cast iron skillet with canola oil. Add butter and fresh thyme at the end, using a spoon to baste. Rest and slice. It will be the most delicious and even medium rare EVER.
I also like searing on stove-top, then finishing in oven. I love Beef Wellington but here's a recommendation for a very good, less complicated presentation than Wellington. It might be a little autumn-ish for early September, but it's delicious:
I always find it helpful to look at the Reader Comments for tips I might be able to use.
Since it's already Sunday, depending on where you live, you might be serving the tenderloin right now! Hope your dinner goes well.
p.s. to Leigh - Recipe above is for a 3lb tenderloin so just cut the ingredients in half for your 1.5lb.
... and searing and oven cooking time will be less, of course