I finally could hide the username field so CBSubs can automatically pre-populate it upon registration. However, it populate the username field with firstname + lastname instead of email. How can I make it set the username same as email? My understanding from reading the forum is that it's the default behavior.

I'm interested in this, too. How did you manage to hide it so it's auto-generated?

Just set 'Registration?', 'Required?' to NO in the Field Management, and Community Builder > Configuration > Login field type to 'Username or email'.

Krileon replied in the past that CB should automatically assign the email as the username but this is not the case.

That doesn't make any sense to me.
I am showing First Name and Last Name fields, but am not showing a username field, since I have people logging in using their email address only. I am not showing the "name" field but I *am* showing the first/last name fields, obviously. Still, when I enter a first and last name that already exists in the system, I can't register and I have to change the first or last name to proceed.