mysql admin question

I'm having trouble with my mysql users.
I'm building a site that allows users to register - when they register they are added to my user table, for database books.
When a user tries to update their account I get an error saying they don't have access. I'm assuming this means they don't have update ability.

My question is - should I be adding them to the mysql user (admin) table and doing a grant command to give them access?
- in my mind this would create security problems with people ftping in and messing with my db.
If not should I be doing a grant command to give them update ability on the database books? How do I handle this situation.

Also - how do I restrict access so they can't log on twice from different locations?
I also want to have a guest account and allow lots of guests to be logged on at once. How do I handle that.

Thanks

Sorted !

For anyone else with the same question:

Users on your site are different to db users.
Use db_connect to connect to the database - using db_user. Grant that db_user update, select, insert and delete privileges on your database.
Then when a site_user logs into your site they all connect to the db via that one user.