You can set the project folder members by clicking "More" on the cover of a project folder and going to Settings > Team Member. When a team member has been set to be a project folder member, all projects contained in this folder will be accessible to this team member. (Note that the creator of a project folder is automatically granted membership with the role he/she has had before.)
Project Folder Permissions
Project access permissions differ between members of a project folder and members of a project. When a user becomes a member of a project folder, he/she can also view all projects and sub-project-folders in this project folder. On the other hand, a user that has joined several projects separately will only see the projects he/she has joined, and the parent project folders will usually appear grayed out, signaling that the user doesn't have permission to access them.
Nest Project Folders
Mockplus Cloud now allows you to create nesting project folders, which means you can create a sub-project folder within a project folder to structure your projects better. Sub-project folders inherit all members of their parent folders.