"sentence" "to be in on" - to be well informed about, to know about "he is in on all the latest fashion, music..." "he is well informed and keeps in on whatever is happening in his profession.....

You've nearly asked this same question before :-) http://www.italki.com/question/270060 My previous answer was: To be "in on it", means to know about a plan that is only known to a select group. I could apply to a crime or a surprise birthday party for example. It doesn't necessarily imply direct involvement, it's more about the knowledge of a secret plan or scheme. So in the case you're asking about here, the "plan" is the prank or joke they were playing on Tom Cruise. They all understood what was going on but he didn't. Thus they were "in on it" whereas he wasn't.

It's used fairly often to refer to secrets or secret plans. If you are in on a secret, you know about it when others do not.