how to start a private group on facebook?

[Marwan]

When you create a group, you can completely control its privacy. There are three different access levels:

Open: For "global" groups, everyone on Facebook can view the group and join. If the group is exclusive to a specific network, only the people in that network can view the group or join it. All content (e.g., photos, videos and discussions) is visible to anyone viewing the group.

Closed: For "global" groups, everyone on Facebook can see the group, but the administrators must approve all membership requests or personally send invitations. If the group is exclusive to one network, only people in that network can view the group or join it. Only group members can view its Wall, discussion board, and photo or video content. Non-members can view its Info and Recent News. If you are not a member, you will not receive stories about closed groups.

Secret: These groups cannot be found in searches or be viewed by non-members. The name of the group will not display on the profiles of members. Membership is by invitation only. Non-members will not receive stories about secret groups.
Group administrators can change these options at any time by clicking "Edit Group" below the group photo.

When someone is in the same group as you, this does not allow them to see any more of your profile information than your privacy settings allow.