Once you have added a user to your organization you assign them administrator privileges. To do this, sign in to account.protonmail.com and go to Organization -> Users.
Under Actions, click the arrow pointing down next to the user you wish to promote to admin status and select Make admin from the dropdown menu.
On mail.protonmail.com, you can do this by navigating to Settings > Addresses/Users > More > Make admin.
Once the user is assigned as an admin, they will need to login into their account -> activate their admin status by entering the Organization password that you chose when you created the organization.
If the user is already an admin, you will see the option “Revoke Admin” from the same dropdown menu.