Hi ​@SteveGPL, as soon as you start working with sub-calendars suddenly permissions, visibility, etc. can get really confusing really quickly. Calendly can only work with calendars that Microsoft exposes to your mailbox as a shared sub‑calendar with edit access. Group calendars don’t always appear in that list. Before anything else, please make sure the target calendar is shared directly to your mailbox with at least Editor/Delegate permissions.

If your Group calendar can’t be shared that way, there are a couple more options. 

• Share a shared/resource mailbox calendar (room/equipment or shared mailbox) to your mailbox with full read/write permissions so it appears under sub‑calendars, then select it under Check for conflicts in Calendly. 
• Or, give that shared/resource mailbox its own login and connect it as a separate Calendly user (useful for collective/team scheduling).

A couple things to remember, Calendly can display only the first 10 sub‑calendars Microsoft returns and can check conflicts against up to 4 of them. Also, you may have to refresh your calendar once you’ve updated permissions. If a shared calendar isn’t appearing, re‑share it via Outlook on the web (not the desktop app) and grant at least Delegate/Editor permissions, then refresh the connection in Calendly.

Here are a couple Help Center Articles with more details that can walk your through these processes as well as give you best practices. I hope this give you some clarity. Good luck, and let us know how it goes!

How to connect you Office 365 or Outlook calendar

How to manage multiple calendars and email addresses