When you get invited to a Project, you will see a big blue Invitations bar under the toolbar. Click on the bar to get the list of your pending invitations. On the next pop-up window, choose whether to join or not the Project.

IMPORTANT: your role in the Project depends on the invitation you received. You can't assign yourself a different role than the one you got originally. 

A user can become part of a Project:

Nonetheless, a user can be part of a Project with no access to Screening Rooms or Delivery Boxes.

You can leave a Project at any moment unless you are the last user in the Project itself.