The user who invites a new participant can give them access to the folders of his/her choice, so that they will be able to consult them.
Warning : By default, a new participant on the project has access to no document.
1) Open the participants menu in the entire project
2) Click on the manage icon on the top right corner
3) Click on folders visibility
4) Click then on the edit icon on the top right corner of your screen, to be able to give the participant(s) of your choice access to some folders. Do not forget to save your changes!