Here's how to change your group from public to private:
1. While in the group, click on 'More Admin Tools.'
2. Click the "Privacy Settings" link.
3. Scroll down the page page until you see the following question: "Do you want to approve members before they can join your group?"
4. Select the "Yes" option and type in any information you want to ask of potential group members.
5. Click the "Save" button at the bottom of the page.