This would add the user 'newuser' with the password 'thepassword' and then give the user access to backenddb from any host with privelages to select, insert, update, delete, create and drop. Its not amazingly hard to userstand the bulk of it. So you'll be able to change anything you want or don't want quite easily.
This will need to be done as the root mysql user.
Code:
mysql> use mysql;
mysql> insert into
-> user (host, user, password)
-> values('%','newuser',password('thepassword'));
mysql> insert into
-> db (host,db,user,Select_priv, Insert_priv, Update_priv, Delete_priv, Create_priv, Drop_priv)
-> values ('%','backenddb','newuser','Y','Y','Y','Y','Y','Y');
mysql> quit
Ive never used the linux mysql util, but I am familer with mysql, and it all looks right from what I've learnt.
hope thats what you ment.