Locking content inside a group

Background on the site:

youhaveihave.com is a website where college kids can register by school and trade items/services with fellow students in order to minimize unnecessary expendatures.

any thoughts on how to hide the content a user created from anyone other than the people in their group(you can use the assumption that the user is only part of a single group) which would allow me to use the classifieds plugin but only allow a user to see the ads posted by those in their group