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Hello, I am also confused by the effect of the admin controls.  Our organization is comprised of several different email domains.  We need all of these users to be able to communicate with each other, but not anyone outside the company.  I have tried enabling/disabling the "Allow off-domain co-workers?" setting, and there does not appear to be any change. 
After reading the explanation KB (link below) I still do not feel i understand these options.  It states that disabling the option will cause a "co-worker" to become a "guest".  Whats the difference?  
https://support.ringcentral.com/s/article/8514?language=en_US
Is there a more in depth document that explains the options in the glip admin console?
Please advise.

Hey Jeff,

No whitelist yet for specific domains, but thanks so much for the awesome feedback. This feature ask is noted by our Enterprise team and they will consider it in a future roadmap plan.

As a friendly reminder:  today you can turn off external communication outside your domain entirely (on/off) in the Glip Administration page. Just look for this Company Settings section, there are a few controls there for invites and sign ups -- based on your desired needs just select the controls you want on.