You can use the embed code on YouTube's site to make a BB code so users can display YouTube movies inside of their posts:
Admin CP -> Custom BB Codes -> Add New BB Code
Title - YouTube
Tag - youtube
Replacement:
Example -
Description - Use the movie code from the YouTube URL in your browser. For example,
http://youtube.com/watch?v=JFwCCL0Vh6U
DO IT MAN!!!!!!