Love camping. We make our tent super pretty and cosy so it's more like glamping really.
My advice is forget sleeping bags, no one wants to be trapped in a bag, take your duvet from home, so toasty and warm. Take a hot water bottle if you get cold.
Take headlights, Aldi do them for £2.99, you will look like a miner but you won't care.
Buy an additional headlamp and put that on your 10 litre water butt (can get these from ebay for a few quid), you have water on tap (invaluable) and the headlight around the water butt creates a lovely, ambient light in your tent.
I would buy a two ring stove if I were you. Otherwise, your eggs will be cold by the time your beans are ready or the kettle has boiled.
Take some solar fairy lights and bunting for the tent for inside and outside, it makes it so cosy.
Windbreakers for creating an area outside the tent, great for privacy and also keeping space between you and other tents. Some campers are massive space invaders. This method keeps them at bay
Get a tent you can stand in, with a separate area to sleep and sit. All these little extras, which don't cost much, could be the difference between everyone loving the experience, or not. As you aren't sure whether everyone will enjoy camping, go second hand where possible.