for_id  for_name       for_des           for_mem
  1      Forum 1    Description 1     mem1@email.com, mem2@email.com
  2      Forum 2    Description 2       null
  3      Forum 3    Description 3     mem1@email.com

mem_id mem_name     mem_email ...
   1     Jane    mem1@email.com
   2     Jack    mem2@email.com
   3     Smith   mem3@email.com

<? $data = explode(',',$row[for_mem]); ?>
<div class="col-md-9">
    <select name="for_mem" id="" multiple class="form-control">
        <?php foreach($data as $key => $value){?>
        <option value="<?php echo $value; ?>"><?php echo $value; ?></option>
        <?php } ?>
    </select>
</div>