You can restrict users from logging in simultaneously with the same username (handy for commercial eLearning, where you want to avoid account sharing), and even ask for Captcha and / or
email verification before letting a new user sign up.
Not exact matches
At the very least, Twitter could require users to respond to an
email or enter a
verification code
before they're allowed to tweet from new accounts.
At the very least a phone and
email verification is needed
before approving a member.
All members must complete
email verification and profile approval
before their profile is made public to members.
PHMSA verifies and authenticates new users
before providing HMIS access by requiring written or
email verification of an official source of the request.
If you haven't commented
before, the first time it will ask you for your name,
email (just for
verification that you are a real person) and your comment.