Signup site should not display more than e-mail

[totto]

With the recent updates, all fields were added to the signup page (see http://demo.getwhydah.com/sso/signup)

People seldom sign up when presented such a gui, unfortunately, it needs to be subdivided into multiple steps. Here are some alternatives:

Hide other fields than username and present an "advanced signup" button

Multi step process:
2.a. Enter username -> Receive e-mail -> On set password also prompt First name, last name and phone.
2.b. Enter username -> Enter first name, last name and phone -> Receive e-mail -> Set password

2.b Risks loosing the user already in step 2, therefore I would prefer 2.a.