Advertisers, but without permission to do anything else

I have a multisite with buddypress. Using classifieds on each site. I would like advertisers to be able to submit advertising to appear on the site they are on, but I don't want these advertisers to be members of the site where they can join discussions or post anything else to the site. Can I do this by creating a user role 'Advertiser'? How would I go about this?