02-25-2018 05:17 PM
11-30-2018 04:27 AM
12-04-2018 01:49 PM
I don't believe you can turn an existing group into an external group. But, you can create a new external group, use the "Share" link to get comments from an existing group into the new external group, and then archive the old group.
It ain't easy, but it can work.