As a Team Manager, or a user with the "Case Manager" permission, perform the following steps to create a new case and/or assign access to an existing case.
Click "Cases" in the top blue menu bar
Click the case you wish to edit, or click "Add Case"
If you are creating a new Case, enter the details, and save the case. Then click on the case.
Click the "Permissions" tab in the case slide out panel on the right
Add a new user to the case by selecting them from the "Add User" drop down
New case participants will be made "Viewers" of the case
Move the case participant up to "Editor" or "Owner" section if you like
There is currently no difference between these roles, and they will be modified in the near future