Regarding the space available in the diner, when we had a family of 5 to seat on the A-Train, we would try to offer options. If the party consisted of two adults and 3 small kids, it was possible to seat the 3 kids facing Mom & Dad, so all 5 could fit if that arrangement was OK with the family. We have even had 6 at one table on a few occasions, but this is pretty crowded and we, as service attendants, would never insist on it. Usually, parties of 5 adults would be seated with 3 at one table and two side-by-side at the table across the aisle. The extra seats would then be filled with other travelers because all seats were used, as our dinner and breakfast seatings tended to be fully reserved. On rare occasions, the party of 5 would show up after most seats had been filled, and we would then have to split up the party or ask them to eat at a later time. This was a less-than-optimal solution, but it was occasionally the only option available.
So I recommend that you all meet in the lounge in advance so that you can enter as a group, and try not to be the last ones into the diner. That doesn't mean you have to charge in like the 7th Cavalry; just try not to be last.
Tom