Follow The Steps-:
1) At First, Simply Download and Install Facebook Social Toolkit in your Chrome browser (PC Only)
2) Then Simply Click on Facebook Social Toolkit Icon From Chrome, and Login with your Facebook Account.
Then Select the option of Add All Facebook Friends as Group Members.
3) Then Enter Group ID of your Choice, In which you want ot add all of your friends, and also enter time delay in seconds. I recommend to use 10 seconds time delay for prevent your account from blocking issues.
4) Then Simply Click on + Add Friends button from there, and you are done. Now it will Start the process, and you can do other works now in new tabs.