You could create a special circle for your group and lock your posts so that only people in the circle can see them, or create a page and limit the posts on the page to the circle members.

You would assign one of you to be the admin and controlling the circle (sort of like in FB).

Every time you add people to your circle you will need to notify the rest of your members and re-share the circle with them or have them add on the new members by name. 

Any more suggestions from anyone?

Take action and have a great day,
Duru