Surprisingly, it would seem that you can't share with confluence-users. Jira-users, yes, so Default groups are fine, site-admins, yes, so Default admin groups are fine. Thankfully we use SSO, and all those groups are available for sharing.
Doesn't look like you can do that with confluence-user and confluence-administrators. But you could create another group and give it the same users as confluence-users and it should work.
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