To add co-organizers without sharing a calendar, this also depends on your meeting type.
You can grant someone delegate access in Outlook. Delegates can receive and respond to e-mail messages and meeting requests and responses on your behalf. You can see detailed instructions on how to enable delegate access and change delegate permissions directly on this link.
Another more convenient way to create is Teams Meeting. For Teams meetings, there are three roles to choose from, and you can grant them permissions directly. Where co-organizers and presenters share most organizer permissions, and you can change roles before and during meetings. After sending the meeting invitation, go to the Teams calendar, double-click the meeting and select the meeting option, then you can assign the role.
Hope this helps!
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