Put this in the BuddyPress theme /registration/register.php file:
<label>Account Type:</label>
<select>
    <option value="optiona">Option A</option>
    <option value="optionb">Option B</option>
    <option value="optionc">Option C</option>
</select>
 
 
Put the following code in your theme's functions.php
 
<?php
/* Add sign-up field to BuddyPress sign-up array*/
function bp_custom_user_signup_field( $usermeta ) {
	$usermeta['signup_type'] = $_POST['signup_type'];
 
	return $usermeta;
}
add_filter( 'bp_signup_usermeta', 'bp_custom_user_signup_field' );
 
/* Add field_name from sign-up to usermeta on activation */
function bp_user_activate_field( $signup ) {
 
	update_usermeta( $signup['user_id'], 'signup_type', $signup['meta']['signup_type'] );
 
	return $signup;
}
add_filter( 'bp_core_activate_account', 'bp_user_activate_field' );
 
function synchro_wp_usermeta($user_id, $password, $meta) {
	global $bp, $wpdb;
 
	$type = $meta[signup_type];
 
	update_usermeta( $user_id, 'signup_type', $type );
 
}
add_action( 'wpmu_activate_user', 'synchro_wp_usermeta', 10, 3);
 
function synchro_wp_usermeta_blog($blog_id, $user_id, $domain, $path, $site_id, $meta) {
	global $bp, $wpdb;
 
	$type = $meta[signup_type];
 
	update_usermeta( $user_id, 'signup_type', $type );
 
}
 
add_action( 'wpmu_new_blog', 'synchro_wp_usermeta_blog', 10, 6 );
 
// change user role on registration
 
function register_role($user_id, $password, $meta) {
 
	global $bp, $wpdb;
 
   	$userdata = array();
   	$userdata['ID'] = $user_id;
   	$userdata['role'] = $meta['signup_type'];
 
   	//only allow if user role is my_role
 
   	if ($userdata['role'] == 'optiona' || $userdata['role'] == 'optionb' || $userdata['role'] == 'optionc' ){
      	wp_update_user($userdata);
   	}
}
 
add_action('wpmu_activate_user', 'register_role', 10, 3);
 
function register_role_blog($blog_id, $user_id, $domain, $path, $site_id, $meta) {
	global $bp, $wpdb;
 
   	$userdata = array();
   	$userdata['ID'] = $user_id;
   	$userdata['role'] = $meta['signup_type'];
 
   	//only allow if user role is my_role
 
   	if ($userdata['role'] == 'optiona' || $userdata['role'] == 'optionb' || $userdata['role'] == 'optionc' ){
      	wp_update_user($userdata);
   	}
 
}
 
add_action( 'wpmu_new_blog', 'register_role_blog', 101, 6 );
?>