<?php
function my_custom_section_registration() {
	?>
		<div id="sa-interest-opt-in" class="register-section checkbox">

			<label><input type="text" name="my_custom_input" id="my_custom_input" /> Label text.</label>

		</div>
	<?php
}
add_action( 'bp_before_registration_submit_buttons', 'my_custom_section_registration' );

/**
* Update usermeta with custom registration data
* @since 0.1
*/
function my_custom_registration_fields_handler( $user_id ) {

	if ( isset( $_POST['my_custom_input'] ) ) {
		// Do something
	}
}
add_action( 'bp_core_signup_user', 'my_custom_registration_fields_handler' );