wpmem_user_upload_dir

$args = array(
    'user_id'   => $user_id,
    'wpmem_dir' => 'wpmembers/',
    'user_dir'  => 'user_files/' . $user_id,
);

/**
 * This example makes a completely custom path for 
 * user file uploads based on the specific user ID.
 */
add_filter( 'wpmem_user_upload_dir', function( $args ) {
  
    // Change default directory if user ID is "10"
    if ( 10 == $args['user_id'] ) {
        $args['wpmem_dir'] = 'special_dir/',
        $args['user_dir'] = 'special_sub'
    }
    
    // Make sure to return $args.
    return $args;
});

/**
 * Add a random hash to the user upload directory.
 * 
 * Change the default name of the user upload directory.
 * This example completely removes the user ID value from the
 * directory name, leaving it as a completely random string.
 * 
 * Change $hash_len to set the length. Example creates a random
 * string 36 characters long.
 */
add_filter( 'wpmem_user_upload_dir', function( $args ) {
    
    // How long of a hash?
    $hash_len = 36;
    
    // Check if user already has a directory hash.
    $hash = get_user_meta( $args['user_id'], 'wpmem_file_dir_hash', true );
    if ( ! $hash ) {        
        /*
         * If there is no existing hash, we need to create one.
         * 
         * To make sure it is unique without having to do a for/while loop
         * while checking the db, we can use the user ID in the string.
         * Since the random hash is already long, inserting the user ID
         * does not degrade the randomness.  Theoretically, one could just
         * add the user ID without this step, but I like to have everything
         * being the same length so that a one digit user ID results in the 
         * same directory name length as a four digit user ID.
         *
         * This example makes it user ID + hash.  For example, where the 
         * user ID is "234", the resulting direcotry would be like this:
         * 
         *     234LXd2B8SE31u19TAS0SnwQW6ji
         * 
         * For ultimate randomness in this example, consider switching to
         * add the user ID to the end rather than the beginning.  I did it
         * at the beginning to allow a user with file system access to be
         * able to browse the directory by user ID if they understood the
         * directory name construction being done here.
         */
        $uid_len = strlen( $args['user_id'] );
        $hash = $args['user_id'] . wp_generate_password( ($hash_len-$uid_len), false, false );
        
        update_user_meta( $args['user_id'], 'wpmem_file_dir_hash', $hash );
    }
    
    $args['user_dir'] = $hash;
    
    return $args;
});