First of all, the MySQL documentation is available
here. Just click the "Documentation" option. I strongly suggest using the "Searchable - With user comments" area.
For a direct answer to your question, though, you can use the GRANT command. GRANT will create a user if one does not exist. So, to grant access to the database "foo" for user "userman" with password "itsme", you would issue this command:
GRANT ALL PRIVILEGES ON foo.* TO 'userman'@'%' IDENTIFIED BY 'itsme';
This will let that user do anything they like to the database. If you need more restrictive permissions, go take a look at the documentation.
-- Kintar