BuddyPress.org

Just make a child theme: https://codex.buddypress.org/how-to-guides/building-a-buddypress-child-theme/ The file you’ll want to edit in your child theme is in the default theme folder under registration/register.php

You may also want to look at this thread which will redirect people to the register page if they go to one of your other pages directly: https://buddypress.org/community/groups/miscellaneous/forum/topic/splash-start/