To add athletes to multiple sessions you will first need to select the sessions you want to add them to.
Once you have selected the sessions just press Assign Athletes at the top of the page.
Below the list of sessions there will be a list of athletes to add to the sessions.
Pick the coaches you want to add to the session
You will see a list of sessions at the top of the page. At the very bottom you will have a list of coaches. Select the coaches you want to add.
Select the permissions for the coaches
Assign & Add Permissions will add the selected coaches to the sessions.