This is a BuddyPress gist. Using it in your active theme will allow you to manage a regular WordPress user meta in the registration form. Just above the profile fields form, you'll see a new text input 'Usermeta'. You can customize the different references to a random user meta i used (bp_cr_usermeta) by your own.

bp-customize-registration.php

<?php
// Exit if accessed directly
if ( !defined( 'ABSPATH' ) ) exit;


/****

You should paste this into a file (in the example below : bp-customize-registration.php ) 
in your active theme's folder, then depending if your theme is a child theme or not you should
use this code in your functions.php to include the trick

not a child theme
require( get_template_directory() . '/bp-customize-registration.php' );

child theme
require( get_stylesheet_directory() . '/bp-customize-registration.php' );

****/

add_action('wp_head', 'bp_cr_css_fix');

function bp_cr_css_fix() {
  if ( !bp_is_current_component( 'register' ) )
		return;
		
	?>
	<style>
	.standard-form #bp-cr-usermeta-fields{
		float: right;
		width: 48%;
		margin-bottom:1em;
	}
	</style>
	<?php
}


add_action( 'bp_after_account_details_fields', 'bp_cr_usermeta_fields' );

function bp_cr_usermeta_fields() {
	?>
	<div class="register-section" id="bp-cr-usermeta-fields">
		
		<h4>Usermeta Profile</h4>
		
		<div class="editfield">
			
			<label for="bp_cr_usermeta">Usermeta</label>
			
			<input type="text" name="bp_cr_usermeta" id="bp_cr_usermeta" value="<?php bp_cr_usermeta_fields_edit_value(); ?>" />
			
			<?php do_action('bp_cr_usermeta_fields');?>
		</div>
		
	</div>
		
	<?php
}

/* if the user made something wrong in registration form, we'll print the posted value */
function bp_cr_usermeta_fields_edit_value() {
	echo wp_kses( $_POST['bp_cr_usermeta'], array() );
}

/*
Here we're adding our regular usermeta to BuddyPress/WPMU $usermeta array
*/
add_filter( 'bp_signup_usermeta', 'bp_cr_signup_usermeta', 10, 1);

function bp_cr_signup_usermeta( $usermeta ) {
	if( !empty( $_POST['bp_cr_usermeta']) ) {
		// let's strip any tags..
		$usermeta['bp_cr_usermeta'] = wp_kses( $_POST['bp_cr_usermeta'], array() );
	}
	return $usermeta;
}

add_action( 'bp_core_signup_user', 'bp_cr_signup_user', 10, 5);

function bp_cr_signup_user( $user_id, $user_login, $user_password, $user_email, $usermeta ) {
	if( !is_multisite() ) {
		/* if multisite we need to update the user meta.
		For multisite config, we'll take care of this by playing with {$wpdb->base_prefix}signups
		once the user has activated his account*/
		
		// let's add a filter if someone wants to apply some sanitizing treatments..
		$bp_cr_usermeta = apply_filters('bp_cr_signup_user_usermeta', $usermeta['bp_cr_usermeta'] );
		
		// finally we're adding the usermeta
		if( !empty( $bp_cr_usermeta ) )
			update_user_meta( $user_id, 'bp_cr_usermeta', $bp_cr_usermeta );
	}
}

add_action( 'bp_core_activated_user', 'bp_cr_activated_user', 10, 3 );

function bp_cr_activated_user( $user_id, $key, $user ) {
	if( is_multisite() ) {
		/* this time, we only take care of multisite configs
		as on single site config, the usermeta is already stored */
		
		// let's add a filter if someone wants to apply some sanitizing treatments..
		$bp_cr_usermeta = apply_filters('bp_cr_signup_user_usermeta', $user['meta']['bp_cr_usermeta'] );
		
		// finally we're adding the usermeta
		if( !empty( $bp_cr_usermeta ) )
			update_user_meta( $user_id, 'bp_cr_usermeta', $bp_cr_usermeta );
	}
}

Thanks!