My #1 piece of advice is to get all of the accounts, profiles and connecting as Friends done *before* you start making or linking hotel and dining reservations and tickets. A little bit of time spent up-front to get everything set up and linked properly makes life MUCH easier down the road.
First, decide who in your group will want/need their own My Disney Experience account. Are you multiple families? Do some have young children? If so, then a "head of the family" person should be designated for each sub-group. Have them create a login account on MDE, and then they can create "managed by them" sub-profiles for those they are in charge of. If there are many adults, it might be easiest/best to have each adult create their own MDE account.
Now, once those are set up, you need to link them all together. Send a friend-request to one of the group heads via your MDE account. Once they accept, you can also add all of their sub-profiles by using the "add by mutual friend" feature. While you should link to everyone in all groups, at the very least people should be connected to those they'll be in WDW with at the same time.
Oh, and for a large group like this, I suggest you set the privacy setting for each person to "can view only shared activities" rather than "can see all". If people can see ALL activities it gets really confusing when looking at your itinerary because you can see things listed that you're not even doing, like other peoples' dining reservations. On my upcoming trip, I have two people joining us for the first part of the trip, two for the last part with a day and a half overlap in the middle. Since they're set to "only shared activities", they'll only see dining reservations for the ones they'll be there for.
Once all people are linked and all friend requests are accepted, you're ready to start making/linking reservations and connecting them to the people who will be using them. For the people who have their own MDE accounts, they can go into the MDE site or app and see all the events they've be linked to.
Also consider using the "Notes" function. You can easily set meeting times and locations, give an overall plan for the day (I have a Note in my profile of "Food & Wine Festival" for the two days we're doing that in November, as well as our water park day at Blizzard Beach). In some places in MDE the Notes get shoved to the bottom of the day's listing of plans, but on the MDE app main page the Notes get sorted properly by time of day.
-Rob