Allow users to register using only an email address, ie. without username. The user, once registered then may log-in using their email address and password for authentication.
A username is generated and assigned based on the user name part of the email address. Subsequent users who have the same name (email prefix) are numbered sequentially. For example, "johndoe@exampleA.com" would be assigned username "johndoe", "johndoe@exampleB.com" would be assigned "johndoe001" and so forth.