Recently Microsoft have added external groups to Yammer. Within Yammer you might have seen the following message:
So now you can use yammer with people who work for other companies that use Yammer without them having to see all of the other groups within your companies Yammer.
When you use Private groups you can even have more control over who can see what.
So I tried to create a new group and it is as simple as filling in the form below: