When a user is signing-up at your site and you do not want to ask more information from user at this time then you should have to get in touch to Joomla Registration Action of Joomlaxi Forms.
This is a pre-installed action of Joomlaxi Forms(Free Extension), to take a look at Joomlaxi Forms click this link http://www.joomlaxi.com/support/documentation/item/joomla-registration-action.html
This action provides you the following ways by which a user can register on your site:
+ By email-id only.
+ By email-id and password.
+ By username, email and password
+ By name, username, email, password
+ By name, username, email, retype email, password, retype password