An Exchange user wanted to know how to share a secondary calendar in an Exchange mailbox.
It's simple: right click on the calendar folder and choose Share > Share Calendar.
This will open a sharing invitation. Depending on the policy settings, you may be able to choose the level of details you are sharing. You will be able to give the person(s) read/write permissions (default is read-only). When you click Send, the calendar will be updated with the necessary permissions.
Calendar Sharing Permission Error
When you share a Calendar in your Exchange mailbox and receive an error message that she can't share the calendar, it's either due to a sharing policy set by your administrator or a problem with your autocomplete cache.
Policy does not allow granting permissions at this level to one or more recipients. Please select another permission level and send the sharing invite again.
If you can share it using Outlook on the web, the policy settings themselves are correct. If you are unable to share it using the permission level you desire, you will need to speak to your Exchange administrator.
If the problem is on the Outlook side (and not a policy issue), delete the autocomplete entry for each recipient then select the recipients from the Global Address List (GAL). The error occurred because the autocomplete entries that came up as you type the recipient's names was their SMTP email addresses, not resolved to their GAL entry, and Outlook thinks they are external addresses and your policy does not allow Calendar shares to be sent to external addresses.