<!--input-group-prepend-->
<div class="input-group">
  <div class="input-group-prepend">
    <sapn class="input-group-text">@</sapn>  
  </div>
  <input type="text" class="form-control" placeholder="Username">
</div>

<!--input-group-append-->
<div class="input-group">
  <input type="text" class="form-control" >
  <div class="input-group-append">
    <span class="input-group-text">@</span>
  </div>
</div>