To add all of your friends to your Facebook group you can simply use a chrome extension that can be found here and install it to your browser. Its free to use and very effective.
Once you have installed the extension simply go to the group in your browser and ensure that you are a member of the group and wait for the page to fully load.
Then you click the new green B button that has appeared at the top of your browser next to the address bar and then click START ADDING.
Now the extension will automatically add 500 friends to your group. You can do this safely every 24 hours until all of your friends are added to your group.