Adding your friends to Facebook groups is now a very difficult business. Previously people wore using scripts in the console and adding about 600 friends within 1 minute and then stopping to avoid banns.
Or people wore using Chrome extensions to add to groups but now all of that has changed.
Facebook have changed the rules and doing either of these things will get you banned and also adding to a group that grows to 5000 in less than a couple of hours will get every account that added to the group completely banned and also every account that trys to add to that group from then on banned.
The group actually becomes poisonous and any account used for adding to a poisoned group will be banned.
Adding Facebook group members now is very hard and takes a lot of time and effort.