No doubt film stars move around in the same rareified circles and get to know each other at least at a distance, perhaps some time before they meet face to face. However, rumour has it that Brad Pitt;s agent got together with Jennifer Aniston's agent and set up a date for the couple.
It seems very likely that they had met socially before this. After all, they were both major celebrities and will have attended the biggest events and so seen each other. And how to ignore someone who is the biggest star, as Pitt was.