You are going off season, it will be hotter and more humid but being from NC you are probably used to that more than I. It will be less crowded and for things like car rental negotiate noting it is off season and they will have cars sitting around. The water will be warmer and clearer, and the odds of a hurricane are low but still something to think about. It seems you have your planning relaxed which when herding cats is good. As mentioned and you probably already know this but you are in a great walkable area. There have already been some great suggestions so far, so noting the budget friendly comment here are my suggestions, good but not breaking the bank.
Beach days - First off show up by 930am at any of these and you will have your best shot of getting lots of chairs. D's, K as in Karakter, and Marys Boon on Simpson Bay all have good food and last I heard free chairs. Note at K, it is French style where the tip/service is in with the food so when you consider you are maybe adding 3-5% tip it is a bit more reasonable. Just note that drinking there can get expensive. On Orient Orange Fever was suggested, might be tough to get 8 chairs let alone 15 you could call for reservations. Food and drink are reasonable there. Grand Case - While we like Grand Case Beach Club getting 4 or 8 chairs can be tough, let alone more. LTC is great but the price is not bargain. I would suggest Frenchy's, a bit more reasonable on all and probably the best place for a chance at a large number of chairs if you want to try Grand Case which has a great beach. Mullet is right by you, Rosies and Da Watering Hole are more reasonably priced and it is a great beach. The beaches all come and go and change daily, including the beach right there at the resort where you will have free chairs and Tortuga really is pretty good.
Food - Walking distance you have a lot to choose from, Roma for Happy Hour is a great choice. Up in Jordan Village you have a couple inexpensive places that cater to college students along with the tourist and locals. Fat Tony's is good, and La Chingona (real deal Mexican) while mostly for take away it is one of my favorite places on island and inexpensive. Dany's at Cupecoy for Sunset and Beer/Cocktails is another favorite. In Simpson Bay I know people that like to take larger groups to White Swan for Chinese Food and for the 20 somethings and above I highly suggest Huppel The Pub especially on Monday nights. Inexpensive drinks, great vibe and great music. Something you might also want to do as a group is contact Richard at Gutside and have a dinner there, can't get much more local than having dinner with Richard and Lourdes and it is very reasonable. There are lots of places for higher end but these were the easier on the pocketbook favorites that we know of.
With 15 you could probably charter or rent out an entire boat for the day or a sunset cruise like on No1SXM, not a out on a boat person but I am sure others can give suggestions on this. You might also want to consider The 12 Meter Regata and racing each other, a lot of fun that can be done as a group.
Have a great trip!
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