To access a Unity Project on which Collaborate is enabled, you be added to the Organization under which the project was created. If you are already a member of the organization, then verify that you are a Manager for that Organisation.
1. Open the project on your machine by opening up the Editor and when presented with the project window, click "In the Cloud".
Then click on the project that you need a copy of.
2. You will then need to select or create an empty folder. Unity will download and place the current version of the project inside. Once this is done, the Editor will open the project and you will be able to start developing.