Drupal 6 Organic Group member cannot access private Group

Dear All

My drupal is 6.22, I want create private group to member.
If I set the group to public , each member can see the page.
But I set the group to private, all member cannot see the page.

And the error message is

Access denied
You are not authorized to access this page

Is I miss some permission setting?

I want set 2 groups as:
Guest cannot visit 2 groups
Group 1 admin can create post to related group
Another group admin can see group 1 article but cannot post and edit.

How can I do it?

Best Regard
Kitty