How can I configure the membership in a way that I can use the email address as the login name? I do not want to prompt for both login name and email.
I thought about not requiring the email and changing the User Name lable to read "Email", but than I can not use the email functionality of the build in controls.

You can for example create your own Membership provider for that.

You can also build your own user administration page, where you call the
Membership.CreateUser(string username, string password, string email) method and pass the username into the email argument. Or use the CreateUserWizard control.

If you want to use the ASP.Net Web admin site, you can modify the code in the add user page(Inetpub\wwwroot\aspnet_webadmin\2_0_40426\Security\Users\addUser.aspx). The Web Admin tool.

Thanks Fredrik, I think I will use this second option.