You can set or update sharing permissions on an individual event by clicking the Sharing icon located in the top right corner in the Event or Travel item. 

When you hover your mouse over the icon, the Sharing Modal will appear showing you what you share, with who and all the permissions you can select. When you select a certain permission, the current Project Sharing Default settings will always be overwritten for this event.
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When your account is connected to several accounts, you can click the blue account name to get an overview of all connected accounts. Simply click another account to switch.