They can all be separate.
I am about to depart for a trip with 9 people (3 families total in 3 rooms). You can indeed link all the groups together. In my experience the best way to handle this is to have at least one person in each group have a MDE account. For all of the others, you can go ahead and make their MDE account (which is the way to go if they will be returning down the road) or you can create what I will call place holders in the system for them. A place holder is simply a name linked to your group that you can use when you make ADRs or Fastpass reservations but you do not need an email address for them. A placeholder account is linked to the person with the actual MDE account. The placeholder cannot log into MDE or make changes. This is why one person from each room you are booking should have an account.
However, while it does take more time, it is best to have everyone have their own MDE log in.
Once you have everyone in your groups set up the way you want you will need to link everyone together. You log into MDE and goto the Friends and Family tab. There are three ways to search out your travel partners. Find their accounts and link up; this is done through email verification for people of have an MDE account. Once you go through it, have everyone check their email to confirm the friend request. Then you wait for the system to catch up a bit.
Once all of this is done it gets easy, you should see everyone in your friends and family page. From then on out, any reservation you make you will have the option to select people from this list that you want to join you for that reservation.
For ADRs, you will select the number of people that will be joining you for the meal and look for a reservation. If you have trouble finding a table for a large group, some people here have had success breaking up their party and booking two tables for the same time. When they arrive they ask to be seated near each other. I have not had to do this; I have not had any issue booking tables for 9 (Crystal Palace, BoG).
For Fastpass reservations it is the same deal. Once you start booking you will see a list of all of your family and friend that you can select from. My advice for FPs is to go ahead and click the select all option for your three fast passes (assuming everyone will be in the same park) and book the experience. After you have everything locked in, THEN go back and alter the reservations for the people that you want to break out into different rides/experiences. Trust me, it is just easier to do it this way. For example, I booked Space Mountain for all 9 people in my group, but three of us don't actually want to ride it. So after the FP was reserved I went back in and edited that Fast pass for the 3 of us to something different but still close by and still around the same booking window.
The other Fast pass trick that I can pass off to you is this. When you are moving around your Fast passes looking for better times, try this. Book all of your fast passes for that day on the rides you want but for times in the opposite part of the day that you want. So if you want fast passes for the morning then reserve evening times. Then go back and edit the times of your fast passes because now you will see every available time slot (the system typically defaults to hiding times around other fastpasses you have - this way you can see them all).
Good luck
