You can link your reservations. If you want them fully linked (you will be linked for dining but would also be able to see/book port adventures; Palo; onboard activities), you can do it online. Depending on how good friends you are, this might be okay. Even then, I might recommend you wait until after everyone had made their activity reservations before you link as once you are linked, only one reservation can be open at one time to make these bookings. If you don't want to give the other friends full access to your reservation or have full access to theirs, you can call and ask to be linked for dining only. I will warn you, this doesn't always work, we have had times where they have fully linked us even though we have requested to be linked for dining only.
Since you are linked,
DCL will know to put you at the same dining table. You don't have to do anything else as far as that is concerned.
For port adventures, as long as you are on the same excursion with the same departure time, you should be fine. If it is a tour that will have multiple buses (or whatever), they don't break you up based on when you made your reservation, they break you up based on when you check in onboard for the port adventure (as you check in, they give you a character or number sticker and this typically designated which bus you will be on). So as long as you show up at the meeting place/time together, you should be together for the tour.