When choosing a tent for a group camping outing, you have several options. First, you can buy one tent for the whole group. If you go this route, consider getting a six-person tent as opposed to a four-person tent, as that will give you more room to stretch out. The downside to this approach is that these tents are heavier and bulkier. It can be far better for both privacy and weight to get two three-person tents and split the group into two for sleeping. With this option, you can also get better, lighter backpacking tents with aluminum poles that are much more durable than the fiberglass ones typically found in family tents.
Each person should carry her own sleeping bag and pad. If you are camping with a significant other, you can get sleeping bags that can zip together for a nice cozy night under the stars.
If you are car camping, a two burner propane stove, such as the Brunton (see resources), will speed up cooking times and give you a lot more flexibility in your menu options. If you are backcountry camping, carry two one burner stoves. These are lighter and far more compact than two burner stoves, and again give you flexibility in the menu planning.
Headlights should be carried by everyone on the trip, and a battery powered lantern can be nice for the group as well. Carry camp chairs of some sort, such as Crazy Creek chairs. Cascade makes fabric chairs that allow you to stuff your Thermarest into the chair frame; these designs are significantly lighter and more compact than a standard camp chairs.
Bring matches, both to light the stove with and, if local regulations permit, create a fire.
Bring a well-stocked first aid kit with a variety of bandages and dressing, antiseptic pads, gauze wraps, scissors, and pain-relievers.