To add athletes and coaches to multiple sessions you will first need to switch to the List view of the Calendar in the top right corner.
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The next step is to select the sessions you want to add them to.

Once you have selected the sessions just press Assign Members from the Bulk Actions Menu.
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This will open a pop-up window where you can select Coaches and Athletes you want to attend the sessions, as well as allow you to give them permissions to Add Results and Edit the session in the Permissions tab.

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