I run a secret Facebook group. When I need to add a new member, I paste an email address into the Invite Members box on the right panel:
Sometimes it works as expected.
However, 90% of the time I send an invitation, the intended recipients then tell me that they never received a Facebook notification or email about the invitation, and so they remain excluded from the group.
Then, my fallback approach (the only sure-fire way I've figured out how to invite someone into the secret group) is to first connect with them as a "friend" and then paste their email address into this Invite Members field. That always works. (But I don't want to NEED to connect as friends.)
This problem has existed for the past year.
I'm wondering if the few times it has worked have been because of certain Facebook profile/privacy/notification settings or preferences of the people I've been inviting.
If I could narrow it down to understand which settings allow someone to seamlessly receive invitations, I could tell prospective members to adjust their settings first.
UPDATE: The latest invitee said that she did receive an invitation email from Facebook on my behalf, but when she clicked the link, it brought her to a page that said
"Content Not Found. The page you requested cannot be displayed right now. It may be temporarily unavailable, the link you clicked on may be broken or expired, or you may not have permission to view this page." (It doesn't make any sense that an invitation to a secret group would still not let her see the group!)
Then I added her as a friend and resent the invitation, and only then did she appear in the "Members ➔ Invited" list at https://www.facebook.com/groups/XXXXXXX/invited/, and there was a note linking to https://www.facebook.com/help/147058305366686, saying that she is in Preview Mode for up to 28 days or until she accepts.