I like the new look and functionality of LinkedIn Groups. LinkedIn Groups can be an excellent tool to build community amongst association members as well as help promote your association. My recommendation is to keep a group "open" to non-members of your association so that they can get involved at a "subscription level" before signing up for a paid membership.
The template feature allows you to provide an auto-response email when you approve a membership in the group so that you can explain the group etiquette, purpose and tell a little bit about how the group membership works vs. paid membership in your association.
Here is a great tutorial video on how to use the group to build and brand your association and its members. The video also helps group members learn how to use the group productively: