Outlook Calendars for Confluence supports to embed Group calendars directly into Confluence. However, due to restrictions in Office 365, accessing Group channels requires an admin consent and is not enabled by default.
Working with Groups requires the Office Permission: Group.ReadWrite.All and
Steps to enable Group calendars
📂 Open the following URL, login as Office admin and consent the permission for the organization.
Afterwards every user can open Group calendars on his own.