The person that invites us to join her group of users should give us a 6-letter/number code like this:
All you have to do is go to user management part of the platform by clicking on this button on the left:
And then click on the "Add" button in the "SHARED" section:
The final step is to type in the given code to the dialog box that will pop-up:
and you are in! You should now see all shared project in that group every time you click on the "Open Project" button on the left.