Sometimes you need to quickly create test users with the intent of later deleting them. Here's how you can create mysql users quickly and grant them all permissions. This is not recommended as you don't want any random user to change any table but as long as you are confident you will delete the user immediately you can use the following:
CREATE USER 'username'@'%.%.%.%' IDENTIFIED BY 'password';
grant all on *.* to 'username'@'%.%.%.%' identified by 'password';
%.%.%.% means the user can login from any host.