How do I add people to my Private Group

I started a group of our Motor Home club on facebook. After discussion with members, they wanted the group Secret so that no one outside of the group could see the post.
The problem I'm having, is since the group is Secret no one can search to find the group on facebook and find it to ask to join. I'm having trouble adding new members.
I click on "add friends to group" on the right side of page under members and a window pops up to "Add friends to group" but it does not say enter email or name.
I've added email address hoping that they would get an email inviting them to the group.
They never got anything. I get emails and phone calls asking how to get on the group, but I'm having trouble adding them.
I looked all over the net this last month and found noting about adding to a secret group.
Hope you can help.

What's the name and link to your group?