We have just released sign up for agency customers, which is one of the major features in the roadmap.
This feature allows new customers to sign up automatically by appending /auth/signup to the custom domain URL that is used by the agency. Using the sign up form (pictured below), they will create a new user and a new client, assuming that there are available seats left. Moreover, the admin users of the agency will be notified of the new customer.
The sign up feature is not activated by default. To activate it, you need to go to Configurations -> General Settings, and click on the "Allow client sign ups (including for existing customers)" toggle.
As a bonus, this new feature allows you to provide a sign up link for existing customers, too. You can obtain the link for each client by going to Configurations -> Agency -> Clients.
Tell us what you think about this new feature, as well as any suggestions for future improvements.