Please see this article regarding logins::
http://dev.mysql.com/doc/refman/4.1/en/user-names.html
You can grant permissions to individual databases. Only the user you grant said permissions to will be able to access that database. It would be something like::
grant select,insert,delete,etc. on mydb.* to 'johndoe'@'localhost' identified by 'passwordgoeshere';
Use .htaccess for phpMyAdmin security. That way only people with user/pass can access the db via phpMyAdmin and will only be able to see 'their' database.