Hi there, you can add your supplier to the site by logging into its dashboard at http://mpnforum.wordpress.com/wp-admin - and heading over to to Users > Invite New
You'll find instructions here: http://en.support.wordpress.com/adding-users/
WordPress has different user roles, depending on how much access you want to give to each user: Administrator, Editor, Author, and Contributor. You can read more about the different roles here, and decide which role fits your supplier best:
http://en.support.wordpress.com/user-roles/
If you have any further questions, just let us know.