www user with mamp

Hello,
I am running wp under mamp on a mac (lion operating system. It constantly warns that while connected to the internet you should run mamp as www user, rather thatn your own name for security reasons. However, when i try to add www as a user in order to give it access to the relevant files, it says that this user already exists; it is never available for me to select from the list of users.

How do i get round this, and what kind of security risk am i taking by running it under my own username?

Many thanks in advance.