<div class="form-group">
    <label class="col-sm-2 control-label">Addon on the left</label>
    <div class="col-sm-10">
        <div class="input-group">
            <span class="input-group-addon">@</span>
            <input class="form-control" placeholder="Username" type="text">
        </div>
    </div>
</div>