I hope you have gotten your answer to this question, but if not I believe this is what you are looking for. In order for a user to be able to switch from Site to Site they must have access to each site. If they do then when they log into the server they can choose which site they want to view. In Tableau verison 8 it allows you to switch site byt clicking on a drop down box. Not sure how the ealier versions allow you to switch sites.