Limit sharing of a site to internal and external users with existing access.

[axyrium]

Is it possible to set permissions on a site such that existing internal users can share with both internal and external users that already have access to the site, but not share with anyone who does not already have access to the site?

I can't seem to find a settings combination that makes this possible. The closest I can come is limiting sharing to internal people who already have access 

[#2]

Looks like I found a post in the MS forums where MS acknowledged they could recreate the scenario of not being able to share with existing external guests when site is locked down to only sharing with existing guests. forum/all/sharepoint-online-unable-to-share-with-external/cc5f2f1e-19e6-4367-954c-b2c4895bf43d

I have a ticket open with MS but they haven't been able to resolve it yet. Maybe this scenario isn't common? 77 views and no replies yet. Would love to know if others are experiencing this, too.
https://answers.microsoft.com/en-us/msoffice/