Postcron uses Facebook's API (or code) to publish your content, which means we have to follow certain policies.
Since May 2015, Facebook no longer allows third-party Apps, like Postcron, to publish to groups that our users haven't admin rights to. Therefore, being a member does not give you enough permission to publish to a group through Postcron. This policy affects every third-party publisher.
When you click on Facebook groups under Add Accounts, Postcron pulls out your Facebook information and shows you only the groups that you admin. If the one you are looking for is not on the list of social accounts, you most likely don’t have admin rights for that group or you aren't logged into the profile that does, preventing Postcron from obtaining the necessary permissions.