Within the project, in the Members section, you must indicate the users with whom you want to share the project. Here you can grant the permissions you need, either permission to link; only link documents to the project or All permissions; where the user besides linking documents, will be able to enter the project module and view the project, related documents, addresses, attachments and reports, always maintaining the permissions that each user has in his role.
You should also check the permissions the user has. Even if you have shared the project, it may not have all the permissions; you should go to Settings > My company > Roles, section Settings, from here you can give permissions to View, Edit, Create or Delete.
In this same section, you must take into account the Visible Roles, and select the role of the employee who creates the documents, otherwise, even if he/she has permissions he/she will not be able to view them.