We had an 8:10 ADR at Garden Grill- we were let into the park a little after 8. We were seated immediately upon arrival at the restaurant and finished by 8:55. The CM just checked our reservation and let us get in the reservation line at the tapstiles. I thought that as long as your reservation was before park opening they let you in together. I would also try for an earlier time(I just got an 8:20 ADR at Garden Grill for October a few days ago!), but if you don't get it try to get there early and be in the PPO ADR line. They may seat you earlier.