I want to add an email confirmation field during the user registration process.
There are plenty of plugins available to customise registration fields in the WordPress front end. However, for just one or two fields with basic validation logic, I prefer using some custom code in a plugin or in the functions.php file instead of opting for a full-fledged plugin that might unnecessarily bloat my WordPress site.
On this topic, the WordPress docs are excellent: https://codex.wordpress.org/Customizing_the_Registration_Form
But… with BuddyPress, things work differently. BuddyPress completely overrides the native WordPress registration mechanism. Here’s how to do it:
First, create a child theme, and then override the file:
buddypress/members/register.php
The basic WordPress fields are handled around line 70 of this template.