Thanks for the quick reply Chris! All I'm wanting to do is create a new user with the same username and password, both of which I can capture when they submit the user form, everything else can be the default user settings for a new user on vB.
A single login would be a 'nice to have' once I get it under control intially, but for now I don't want a login bridge, or single sign in, just the ability for people who join the main site to be able to login and post on the forums without having to create another account

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If it can be done fairly painlessly then I'll certainly be buying vB to use, as my time is pretty limited at the moment.