How can I change the privacy setting of a Facebook group (500+ members) from secret to closed?
I can't see any option to do so in Edit Group Settings.
As of January 2016: It seems that at some point recently, the Facebook article quoted by AlvaroFG was edited ("about a month ago") and now reports that the cut-off number is 5,000. If you still are under 5,000 members you should be able to change.
You might if you are still under 5k members, as per Facebook rules after a group gets above a certain size (5000 users), you can only increase its privacy level.
Groups with 5,000 members or more:
- The privacy of the group can only be changed to a more restrictive setting (ex:Public to Closed, or Closed to Secret). We do this to protect members of these groups from having their posts shared with audiences they didn't intend.
- Once the privacy is changed to a more restrictive setting, all admins have 24 hours to change it back.
(Thanks Michael Bray)