If it is a resource, yes you can invite it as such. Otherwise, drag and
drop works well for this as long as you have rights to the other calendars.

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Milly Staples [MVP - Outlook]
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After furious head scratching, mar270 asked:
| I want to enter a calendar event in my personnal calendar and have it
| show up also in a project calendar. I have seen in the past that you
| could include the other calendar in the invitee list but can't
| determine how to do that