There are a few things that need to be in place so that you can access the team calendar.
For athletes
Make sure Team Calendar is active in the Environment Settings
The athlete should be a member of the team
For trainers
Make sure Team Calendar is active in the Environment Settings
The trainer should be a member of the team
Administration
Administrators can see all teams but they cannot look into the team calendars of the teams. The Team Calendar navigation is not shown for administrators. They have to be added explicitly to a specific team to see the team calendar.
For individual users you can open them in User Management and look at their Team Memberships section
Activating the individual check boxes (☑) will add the user as athlete or trainer to the team shown in the row. Clicking the team name will open the team management page for the selected team.