Hi Survey-Gizmo Team.
I have a very urgent question. How can I set it that a person sees only one project and can only work on one project?
This section in the article@IS suggested is probably what you want to look into:
Any project that you place in the Limit Access to the Following Projects ONLY section will be the only project visible to that user.
Hope this helps!
This help page seems a little out of date, but might get you pointed in the right direction: