Currently, I recommend PUBLIC. We are wanting to get as many people in as possible. This is the "pond" you are filling that you can "fish" in later with people who are interested in what you help people with.
Even if it is Public, you can still select it so that you approve people who come into your group. And as admin, you can still delete, block or remove people, etc. (However, FB changes things all the time. They used to have 3 settings: Public, Private and Secret. So just be sure if you see this in the future that you check that their description matches what you want.