You can create a data source filter that is controlled by a parameter. Give an initial "Welcome" page that has a sheet built on a bogus data source (excel or something simple). On that page give them a parameter to select their project. The sheet will just look like a little icon that displays an arrow to click. When they click the arrow, a SELECT action will take them to the actual dashboard where the real stuff is. The data source filter for the data source(s) for those sheets will then be limited by the value selected in the parameter.