The 7.2x version of OG leverages the Entity Reference module to create the relationship between the group node and entity. There about 8 possible OG Entity Relationships to choose from in views that upon review seem pretty ambiguous. I just need to figure out which one(s) are required to create a join link when listing all of the groups on the site.
Add a field to your view of the type Content: Group - set the formatter to OG subscribe link - screenshots in this thread on drupal.org:
If you want to use displays you can just create a code field with a link that points to /group/node/$groupID/subscribe/og_user_node using code fields. Otherwise you can fields and Content: Group as posted earlier.