You can override the registration template in a child theme.
Steps:
Copy the registration template from wp-content/plugins/buddypress/bp-templates/bp-legacy/buddypress/members/register.php
Paste it into your theme folder in wp-content/themes/your-theme/buddypress/members/register.php.
Modify the HTML, PHP, or structure in the file as needed.
I also want to edit the registration page and customise the messages on it and the way it looks, so I tried this too. What I did was, I created a child theme in its own separate folder called buddyx-child. Since the steps say to
`Paste it into your theme folder in wp-content/themes/your-theme/buddypress/members/register.php.
I created a folder called “buddypress” in the child theme folder, then in that folder, created a folder called “members”, and copied and pasted the “register.php” file there and made my edits there. I typed some test messages using the paragraph HTML tag <p> and the message shows on the registration page, BUT the visual layout of the page changes. I don’t know why that is when it’s supposed to inherit the CSS from the parent theme. If I move the “register.php” file out of the buddypress/members/ directory and put it directly in my “your-theme” (ie my child theme) directory, then it doesn’t work at all and the child theme just inherits the original registration page.
Anyone with any ideas?