Crystal Reports : Multiple Values within Parameters

Let us consider a parameter named {?UserID} within our Sales Report.
We can allow the end-user to select multiple UserID's by creating our {?UserID} parameter to "Allow Multiple Values". This is a checkbox on the main parameter property page.
This will allow the end-user to add however many UserID's to the report output they want. The "Allow Multiple Values" functionality is also transparent to the Report Selection Criteria.
Example:
{SALES.USERID} = {?UserID} will return every userid the end user has selected.