When you create a meeting request in a secondary calendar, you'll get a message that responses to the meeting request won't tally because the meeting is not in your default calendar:
This meeting is not in the Calendar folder for this account. Responses to this meeting will not be tallied. Do you want to send anyway?
This is the standard warning message when the meeting is not on your default calendar. If you send it, the responses will not automatically update the tracking. Why? Because when Outlook processes the responses, it only looks for the event in your default calendar.
You will be able track the responses on the meeting request by changing the response in the response field on the tracking page.
In order for the responses to be automatically processed, you’ll need to add the meeting to your own calendar. If you want to use a non-default calendar, you can either send it from your calendar and copy the event to the other calendar as a placeholder or manually update the event on the shared calendar as you receive the responses. If the calendar is in a shared mailbox, Group, or Public folder (enabled for email), you can invite the mailbox or calendar to the meeting.
Open the meeting request and switch to the tracking page. right click in the response field and choose the desired response.
The attendance field is also editable on this page, if you need to change invitees between required and optional.