Bootstrap html button without submit form

Here is my code

        <form class="form-horizontal">

          <div class="control-group">
            <div class="controls">
              <button onclick="javascript:add_user_group(2);" class="btn">add</button>
            </div>
          </div>

        <table id="users-table" class="table table-striped" >
          <thead>
            <tr>
              <th>User Name</th>
              <th>Delete</th>            
            </tr>
          </thead>
          <tbody>

          </tbody>
        </table>

      </form>             

I just want to add a new row to the table when I click the add button, but it seems that clicking the add button causes the form to be submitted.

Not just a button with

 type=submit

submit the form. Or does any button on the form trigger the presentation of the form?