For a long time it has been possible to add 600 people to a Facebook from one account within a few moment sand not get banned.
It has also been possible to add a person every few seconds to a group and add all friends without getting banned.
Facebook have now taken some hard measures to combat the artificial growth of groups. Adding too many people to a group in a short space of time will now make the group become poisonous.
This means that after the adding limit has been reached every account that tries to add to the group for the following 5 hours will be banned from adding to groups for 2 weeks plus. It does not matter if it is only one member that you add, Facebook will ban your account from adding.
The safest way to add to Facebook groups is to add no more than 10000 per day, no more than 1000 per account and no more than 1 person every 10 seconds per account.
Buying facebook group members means you have to find a trust worth and well developed seller to deliver your order.
The best in the business at the moment is FBAdded.com They are very cheap compared to the others and they always deliver on time.