generating "assigned" usernames and passwords

I have a need to limit access to part of a site and be able to generate unique usernames and passwords to send out to those I want to have access. They will be sent using PHP mail functions, looping through the addresses in a database, each loop must generate a different and unique, username store it in the database and add it to the mail Users will be able to UPDATE the database to a more easily remember username and password, later, if they choose. I tried to search the archives, but haven't discovered how to generate the usernames and PWs to send out links, tips, suggestions ? thank you for your replies