Default BuddyPress registration form is very simple but you can customize it to Facebook-Twitter type by enabling AJAX check for username availability, password strength, email check, terms and condition box etc.
You must have observed while creating your Gmail account or your Facebook profile that as soon you type username it automatically checks it availability, when you choose a password it tells how strong it is hence it makes registration easy because you don’t need to click “Register” again and again to know that the information you provided is OK or not.
For enabling this feature in your BuddyPress registration form just install and activate BP Xtra SignUp plugin and customize your registration page from Dashboard > BuddyPress > BP Xtra SignUp. You can add MailChimp Integration, TOC block, Email Configuration, AJAX Username Availability, Email Check, Password Strength Meter and Date of Birth Check option.
Note: If the plugin don’t works for your BuddyPress install then go for – http://buddydev.com/plugins/bpdev-username-availability-checker/