RG3 client’s interface does not imply self-registration for security purposes. The only exception is the “Admin User”.
Admin user always has full access to all client’s functionality by default. There can be only one admin user. You will be offered to create admin user upon your first login to clients interface (http://rg3.example.com/AcmeGroup):
Here you need to enter username, email and password to perform self-registration. Upon registration you will be immediately redirected to Dashboard page from where you can either use RG3 as a client or continue configuration: