You'll first need to head to the 'Your Gym' section of your Member's Area and then select 'Share' under the 'Your Friend Access' tab. Your friend will then need to follow the link that you send them to sign up.
Once they've signed up, they'll be able to use their PIN to manage their membership online and train with you at your home gym four times a month. They must enter the gym at the same time as you.