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When installing the app from Atlassian Marketplace, sharing is enabled by default. As administrator you can disable calendar sharing globally within the admin configuration in the app.
In addition to enabling this feature, you can also set the default behaviour. Available options are:
Everyone that is logged in with Office 365
Everyone that is logged in with Confluence
Everyone with access to this page
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Further information as well as the technical reference on shared office items can be found here: