Put this in your loop

<form action="" method="post">

<input type="text" name="Name" value="username1">

<input type="hidden" name="toPost" value="<?echo $post->ID; ?>">

<input type="submit" value="Submit" name="submit">

</form>

and this somewhere out of the loop:

 



<?php



if(isset($_POST['submit'])) //If submit is hit

{

  



  $name = ','.mysql_real_escape_string($_POST['Name']);  // fetch user input from the form. You could use user_id or username

  $toPost = $_POST['toPost'];  // our post_id to insert username





  $result=mysql_query("UPDATE wp_postmeta SET meta_value=concat(meta_value, '$name') WHERE post_id=$toPost AND meta_key='Name'");

  //SQL query: add to field "Name" new username 

      

     //  uncomment for debug

    //print mysql_affected_rows();

   // print mysql_error();





if(!$result) die('Error: '.mysql_error());

else echo 'Inserted row with id='.mysql_insert_id();

}



?>

This will add the value of form to post you chose.
Finally you could insert user_id or his username and delete the textarea