How many people can be in a FairThen group?
A group can have up to 100 people, and there is no limit on how many groups you can make.
The same 100-person cap applies to a single expense, so in practice the group is the limit you would meet first.
If you are splitting something between more than a hundred people, that is an event rather than a dinner, and a spreadsheet will serve you better than we will.