To enforce an Invite-only system where new clients can only sign up if you send them an Invite, go to
Clients > Settings and uncheck the 'Allow new clients to sign up.' checkbox.
In case you do allow new clients to sign up, you have the option of showing a Sign Up link in the top right corner of your website. Go to Website > website navigation
You can also add the sign-up widget to any of the webpages to make it easier for website visitors to start the sign-up process. Go to any web page, click on the Edit button in top left and add the Signup widget
To customize the Client form as well as add required and optional custom forms go to Clients > Forms.