Use JOIN() Function to Display All Members of A Multivalue Parameter

Sometimes you want the report to display all the pamameters that are applied, especially when the parameter section is hided. To do that, you can simply use a text box and and point the value to the parameters. However, to display all members of a multiple value parameter, you will need to use the JOIN() function.

For example, if you have a parameter called Area that filters work items by the Area field, the following expression will allow you to display all the selected areas on the report.
="Area: " + JOIN(Parameters!Area.Label, ", ")

Thanks to a post I found on bokebb.com. Here is the link to it:
http://www.bokebb.com/dev/english/2008/posts/2008112686.shtml