They are working on multi-company groups which are specifically for that case.
Think @SlackHQ single-channel guests.
#facebookatwork

You can invite anyone (with different email domains) to your #facebookatwork,
but they will be able to see everything of your company.

Facebook has 33,000 groups for 10,000 employees. #facebookatwork

Good to hear, the support SSO via Google Apps. #facebookatwork

Looks a lot like Yammer. #facebookatwork