You can only select a single value from a list parameter. However, if you need to select all, you can add another value to the list called "All" and build the functionality around it.
For your current requirement, you should probably use the organization field itself as a quick filter which will allow you to select multiple values from it.
Multi-select Parameters has been on the Tableau wishlist for a long time.
Maybe some day, but today is not that day.