Hit the PS button on the controller and you should see an option to Assign Slots- go there and select the virtual card you want to use, then place it into "slot 1". When the others want to play just do this each time to put their card into the slot.
You can have as many cards as you like on the hard drive- they just need to be assigned to the virtual memory card slot to actually use, just like sliding the actual card into a PSOne/ PS2 memory card slot.