Support intro
Sorry to hear you’re facing problems. 
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If you’re using Nextcloud in a business/critical setting, paid and SLA-based support services can be accessed via portal.nextcloud.com where Nextcloud engineers can help ensure your business keeps running smoothly.
Getting help
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The Basics
- Nextcloud Server version (e.g., 29.x.x):
31.0.10
Summary of the issue you are facing:
Scenario: A calendar is shared with a group. A user which is member of a group “unshares” this calendar.
It doesn’t seem to be possible, to rejoin the calendar for the user. Nor seems to be a possibility for the sharing person to “re-share” the calendar.
Did i miss a possibility to achieve a re-joining or re-sharing of the calendar? This would be much needed.
Thanks for replies!