Add all the PCs (not users) to a CSV file.
Go to Manage > Filters. Right-click on a Folder where you want to create the Filter for those PCs and select New > Filter. Under the "Filter Definitions" section > Explicit Inclusions and Exclusions > Resources included in this filter, click the icon with the blue arrow. Browse to your CSV file and select it. The filter will then contain all the PCs listed in your CSV that have an Altiris Agent on it.