Define "see". For many customers visibility to all projects via the Executive Tab meets the need you speak of and this gets around the whole adding users issue. If your users need hands on, working access to the project then yes, Johns' solution is the best as we do not have an automated forced membership option.
If they truly only need to see a project's data or basic info (those things visible in the Exec Tab) then bump your users up into a modified Executive profile and set the permissions so they can't do anything you don't want them to.
hope that helps!
Steve Thompson | Sr. Solutions Consultant
Daptiv