BuddyPress.org

@stripedsquirrel

The register.php page is a BuddyPress-specific theme file that let’s you more easily customize the user registration page. It is found in the /registration/ directory of the parent theme bp-sn-parent. When it is in use, it takes over the duties from the wp-signup.php page which is a WPMU file. You can use either one, but I find it easier to use the register.php page.

Also, read this thread. The OP created a new Captcha plugin for WordPress that is compatible with version 1.1.x of BP:

https://buddypress.org/forums/topic/si-captcha-for-wpmu-and-buddypress#post-24383

Here’s the direct link to the plugin in the WP repo:

https://wordpress.org/extend/plugins/si-captcha-for-wordpress/