Hello,

Please try the following code:

/**
 * Get affiliates users
 *
 * @return array
 */
function buddydev_get_affiliates_users() {

	if ( ! class_exists( 'UAP_Main' ) ) {
		return array();
	}

	global $indeed_db;

	$affiliates = $indeed_db->get_affiliates( - 1, - 1, false, '', '', array() );

	if ( ! $affiliates ) {
		return array();
	}

	$users = array();
	foreach ( $affiliates as $affiliate ) {
		$users[ $affiliate['uid'] ] = $affiliate['name'];
	}

	return $users;
}

add_action( 'uap_register_form_before_submit_button', function () {
	$affiliate_users = buddydev_get_affiliates_users();
	?>
	<div class="uap-form-line-register uap-form-uap_affiliate_user">
	<?php if ( $affiliate_users ) : ?>
		<label for="uap_affiliate_user" class="uap-labels-register"><?php _e( 'Affiliate User' ); ?></label>
		<select name="uap_affiliate_user" id="uap_affiliate_user" class="uap-form-element uap-form-element-select" tabindex="-1" aria-hidden="true">
			<option value=""><?php _e( 'Choose from affiliate users' ) ?></option>
			<?php foreach ( $affiliate_users as $user_id => $name ) : ?>
			<option value="<?php echo esc_attr( $user_id ); ?>"><?php echo esc_html( $name ); ?></option>
			<?php endforeach; ?>
		</select>
	<?php endif; ?>
	</div>
	<?php
} );

add_action( 'uap_on_register_action', function ( $user_id ) {
	// No checking nonce as action fire after checking nonce.
	if ( empty( $_POST['uap_affiliate_user'] ) || ! function_exists( 'bp_follow_start_following' ) ) {
		return;
	}

	$args = array(
		'leader_id'   => absint( $_POST['uap_affiliate_user'] ),
		'follower_id' => $user_id,
	);

	if ( ! bp_follow_is_following( $args ) ) {
		bp_follow_start_following( $args );
	}
} );

For automatically follow selected affiliate user on the register.

Regards
Ravi