So you mean that you have a WP install in domain root http://yoursite.com and have installed a second WP at e.g. http://community.yoursite.com/ using the same database and the same users table?
When you say standard PHP sign-up form, I take it you mean it’s not the actual WP login.php? So have you tested what happens when you remove that standard PHP form (which could have caused conflict with registration) and go the route of a direct link to the BP registration page slug http://yoursite.com/register or to whatever URL/slug you set up for your BP install?
