Privacy question... Can individual projects be segmented from one another?

We have a client that needs to keep each of their projects, and the researchers for each project, segmented. However, if we set up a single client with multiple contacts, they could go into the portal and see all of each other's projects. Is there a way to keep each project isolated? Any suggestions would be helpful! Maybe the answer is to eliminate access to the client portal? Are there any other thoughts?