If it is a YouTube or Google Video you can embed it into the forum using the technique discussed here:
As far as copyright goes... with both Google and YouTube (now one and the same) copyright always goes back to the video creator. However, users of Google/Youtube extend a world-wide, non-exclusive right to Google/YouTube to display and broadcast the videos. This extends to all users (both publishers and viewers) and permits the embedding of the videos into 3rd party sites. In fact, both Google and YouTube make the embedding code freely available for all their public videos.
Occasionally copyrighted works will appear on YouTube which shouldn't be there (tv shows, movies, etc.). YouTube usually removes these when they are found (which would also cause them to be removed from KH automatically if they appeared here). I would refrain from embedding any videos that might be questionable in the copyright department.
I hope this helps.