Is there a standard implementation of a Users Database?
I suggest that you use typical normalization standards. One user per row.


User ID (incrementing bigint)
User Common Name (to be displayed on
the site)
User Email Address
Password Salt (Unique for every user, inserted when the account is created)
Password (Hashed with the salt - MD5 or SHA1, your preference)
Date Account Was Created


The rest is up to you given your business rules.