There are 3 privacy options for Facebook Groups: Public, Closed, or Secret. When choosing your Group's Privacy Setting, consider these points: Once a Group reaches 250 members, you cannot decrease the Privacy Setting. For example, you can increase the Privacy from Public to Closed or from Closed to Secret but you can't go from Secret back to Closed. You also cannot change the name of the Group once there are 250 members. Here's a chart from Facebook's Help Center to help you decide which option is best:
Another thing to note, is that you cannot invite users to join by email if it's a Secret Group. Secret Groups are also not viewable by web address (sharing the url). So if you want to join a Secret Group, you have to be Friends with someone who is already a member.
To edit your Group Settings click on the ellipsis (...) in the far right corner at the bottom of the cover image. Then select Edit Settings.
When trying to decide on Post Permissions and Approval levels, consider the following: