Just a suggestion/ another option- We have been on a couple of cruises with groups of 15-30 people and the person organizing our dinners in the MDR had a good process. We would meet at a quiet lounge for drinks before dinner ( 5:30-6 ) and our "host" would go to the MDR desk with the names/cabin numbers of whoever wanted to go to dinner together- the MDR staff would then tell him when to come back as a group and we would get the larger tables at the back of the MDR. It may sound complicated but it ran smoothly every night.