The fastest and safest way at the moment to add people to a Facebook group is simply by using a chrome extension called Group Invite All. You can add it to chrome from the chrome store for free by following the link.
Simply add the extension and you will see a nice new round blue button appear next to your address bar. Now adding is fast and easy. simply login to Facebook from your chrome browser, open the group page on Facebook “you must be a member” and then click the new blue button.
A popup window will appear with the adding status telling you how many people are left to process, how many people have already been added and how many people have just been added.
The safest way to use this extension is to only add 400 people per day from your account. you can of course use the extension on multiple accounts by logging out of one account and into another.
Long gone are the days where you have to spend hours manually adding group members.