How to make social groups invite only in Odoo 8

I'm trying to make the groups in the "Social Network" module in Odoo 8 invite-only. The issue is when I set a group to private, any user can still follow it. Is it possible to make these groups private, or am I misunderstanding how these groups are supposed to work?

edit- I have tried making the group "invisible" by setting the privacy to "selected group only," but users not in the selected group can still join.

edit- I ended up doing a complete reinstall and could not reproduce my issue. I believe I may have had accidentally changed something in technical features. I accepted the answer below because that is in fact the correct way to do this.

Solution

There is no concept as "Invite Only" in Odoo. If you have to hide one group from some specific group of people then you will need to

set Privacy as " Selected Group Only "
Then select the group which group can see this group.
For example,
Human Resources/Employee

So only those who are assigned the "Employee Rights" of "Human Resources" will be able to see this group and can subscribe to it.