Organic Groups (OG): Make public group posts visible to everyone (outside the group)
You control the visibility of a post through the Group_Access field. 

Create a content type with Group and Group_Access fields (or add those fields to a built-in content type). Then when users create posts of this content type, they can make the content part of the group and set its Group Access to either "Public - accessible to all site users" or "Private - accessible only to group members." 

Any "Public" content will be visible to anyone on the site, not just group members.