Creating forum registrations automatically from an external site

[Chutzpah]

I've done some searching (both and google and on here) and can't find an answer to this, sorry if there has already been a post about this or the answer is obvious.

We are currently running our main site and vBulletin forum alongside. People have to register to use the features of the main site, and then currently register separately for the forum.

Whereas our main site has been tremendously successful and has over 2000 registrations already, a very tiny fraction (2%) of that has signed up to our forum.

We want to really get the forum going, and I am trying to find out if there is some way to link forum registration with an external site, i.e. when people register on the main site they instantly get registered for the forum too.

I can't find anything about this in the manual or on the vBulletin.com site.

Can anyone help? I can't believe that I would be the first with this problem.

Thanks in advance!