Well Happy Birthday (early!). I've been camping in the park for 18 years. We usually camp at Canyon Village but I think for a group you should go to Grant Village or Bridge Bay. They are laid out in a better way for groups. I don't think any of the campgrounds in the park offer "group" campsites. If you don't mind primitive then Indian Creek is nice too but is first come first served.
Only certain campgrounds offer services. All campsites are primitive with no hookups except for Fishing Bridge and you can only camp with big RVs there. Some campgrounds have dump stations, but not all of them. I know Fishing Bridge, Grant Village and Canyon do. The following are the ones that have showers (pay showers. Runs about $3.00 a shower) and laundry:
Fishing Bridge (has General Store with small diner and limited groceries, small visitor's center.)
Canyon Village (has General Store with diner, groceries, coffee bar. Also dining room and cafeteria and US post office and cabins. Very large and AWESOME visitor's center).
Grant Village (General Store with fast food, very limited groceries, gas station, steak house (kind of cool, right on the lake), visitor's center, and marina).
You can make advance reservations at the following campgrounds. Be sure to do it ASAP! You will need at least 6 months advance to get what you want. Limit is 30 days before July 1st and 14 days after. When you call be sure and tell them how many sites you want and that you need them all together. They will also ask for size of camper/truck.
Madison (no services)
Bridge Bay (no services)
Grant Village (Opens in Mid-July depending on bear activity)
Mammoth (no services)
If Cabins are more your thing you can reserve them at the same place (Xanterra). They are at Canyon Village, Mammoth, Old Faithful, Roosevelt and Lake. They are all very primitive but at least have a bed and shower. No cooking at any of them.
And of course there are the hotels but I don't know much about them. They are just too expensive for us.
If you want a really nice birthday dinner then you should all go to the Lake Hotel dining room. Very good munchies! Will run about $40 a person. When you make camping/hotel reservations ask for the dining room res. then too. They book about 2 months ahead. Well worth it!
Have a great time and I hope this helps. You are welcome to come on over to our YahooGroup and ask any questions you might have. We are all addicted to the park :)
And don't forget that you can get the Senior Passport to the national parks! A one time $10 charge and you get into all National Parks and Monuments for free. Plus you get 50% off camping and more.