Hello Emma!
How exciting! Traveling with a larger party always makes a Walt Disney World vacation twice as fun. My family is a party of 7, and then we often include grandparents or other extended family members. We have never had trouble making FastPass+ selections together as long as we do it all at the same time. I recommend making sure everyone is linked together on My Disney Experience as 'Family & Friends.' That way you can have one primary person who makes everyone's selections. I also suggest making sure all park tickets are linked in My Disney Experience, so you can make your FastPass+ selections 60 days in advance if you staying at a Walt Disney World Resort. The sooner you make them, the more options you have. Seven Dwarfs Mine Train can often fill up, and you don't want to miss it!
The key to making dining reservations together is also to plan ahead. You have plenty of time to discuss what restaurants are your must-do eats! Make a list and be ready to book them 180 days prior to your arrival date. Some restaurants may have to split up the party due to size, but they always try their best to keep the party together. We always had the best luck when making reservations at off-peak times. When the restaurant is not as crowded, then tables can be pushed together to accommodate larger parties. One of my favorite meals was a late breakfast at
Tusker House Restaurant in Animal Kingdom Theme Park. We were a party of 12, and it was during the turnover from the end of breakfast to the beginning of lunch. The meal was very relaxed, we had the most awesome character interaction, and our server chatted it up with us. And when you can eat chocolate croissants and plantains and rice in the same meal, magic happens.
If you have any other questions, please come back and see us real soon at the Disney Parks Moms Panel! Have a most magical grand gathering!