merging member tables
 

I am adding a service to my site which already has vbulletin. When a user signs up at that service, then I would like them to automatically sign up at the forum. Basically I want to somehow merge the user tables and eliminate double sign up, but I'm not sure how to even start with this.

I'd like some guidance such as where I should look for a tutorial, the problems that may occur, incompatibilities, things to look for, etc.

I have absolutely no money and am already boring from colleagues, so paying someone is out of the question.

Thanks for any help.