I'm using the Buddypress Xprofile Custom Fields Type plugin (https://wordpress.org/plugins/buddypress-xprofile-custom-fields-type) and I know it requires a little hacking, which I've done, to get it to work with the current 2+ version of BuddyPress. I've got the HTML5 fields working now on the BuddyPress registration page, but when I go to the Membership registration page it's not working. I found this thread: https://premium.wpmudev.org/forums/topic/x-profile-fields-dont-show-up-on-the-registration-page#post-760258, which seemed to address the problem, but I can't find the referenced bp.registration.form.php Membership file, and so I can't make it work.
Basically, what Membership plugin file do I edit, how do I edit it, and where in my child theme do I place it (folder structure) to get this working again? For BuddyPress, the files were register.php and edit.php within BuddyPress > Members folder in my child theme.