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This help page seems a little out of date, but might get you pointed in the right direction:
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!