newbie - correct use of Grant in mysql

Hello I know this is off topic but I have tried at the mysql newsgroup/mailing list but the just ignored me and you all seem like such nice people :) I am setting up a kind of PHP guestbook that will allow the user to log in and update their details. Should I create a new user using grant for ever (unique) person in the guestbook so that way they only have the rights to alter certain specified fields of their own data or do I create one user for everyone with the right to alter particular fields in the table. I suspect the former but this may give me thousands (Or even more) mysql users - it is a very popular guestbook. Is this normal? Is their a limit on the amount of users? Please let me know if I have got this all wrong :) Thanks in advance for your time Darren Smith dagza@email.com