Did you know that your project folders are color-coded based on your access? If you have access to the whole project, the folder will be blue. When you open the project, you will see all the participants of that project.
If you have access to some boards in a project, but not the entire project, you will see a gray folder. If you only have access to a few boards in the project, you will see the project owner instead of the list of participants when you log in. Then you can request access to the project from that person.
It is important to add collaborators to projects, not just to boards, as it gives your collaborators additional capabilities. For instance, I must be a project member to move any boards I have created into a project.