Enter the email address of the person you want to invite and clickSEND INVITE. A confirmation email will be sent to you when they accept the invite and are added to the plan.
Once each family member joins the family plan, they should be able to enjoy the Premium features with no issues.
However, you'll need to make sure that, if they're accepting the invite on the same device it was sent from, the owner who sent the invite has logged out of their account page first.
Don't forget to log out on all devices and ask your family members each to use their own login details when using the app. This way, everyone can listen to music from their own device at the same time.