I know, but they will be friends of your own cat.
Jiskefet could always get in, even if he lost his collar. He just waited for Catweazle, Steffie or Tuppence to go in and followed close behind. But the other cats LET him follow. NO cat will allow a rival or enemy to follow him that closely through the cat flap.
Besides, you can adjust the time during which the petporte will unlock. I always set it very short, one or two seconds, to prevent other cats from following when mine are being chased. But if you have a very hesitant cat, you can set it up to 30 seconds or longer, just as you wish...