Only allow admins to post on a public Group in Buddypress

Hi,

I need to have the main site group stream to be public so we can share it successfully to social media. But i only want it possible for one site admin to post on it. How do i set this up?

The group is 'YNFA - THE HUB' and the admin user i want to have posting access is 'YNFA' also.

You can access the site to do this at your pleasure throught he WPMU admin panel.

Thanks in advance.