Actually, this was our 3rd GG trip. We did all adr's online (except for the GG events themselves) this time. BUT..... the previous times, we actually DID use group dining and they were more than helpful. Got us all into Coral Reef when the regular dining line could only get us 2 ressies 10 minutes apart.
I don't remember a specific number because I believe I just called wdw-dine and asked for group dining. this was 2 and 4 years ago, so things may have changed but it's worth a shot. All they can say is no.