I encountered thsi trick quite recently so hopefully not too many people will know it.
First I will start a number and the next poster look at it and put the first number that comes into your mind (they must be positive integers between 1 and 1000). Then the next person and the next person etc. Soon a chain will build up but also a pattern - I will post the connection once a few numbers have been posted and you will be shocked at how you missed it.
Ok, so here is the starting number - 535.