how to do a line break in Bootstrap 3.3.7

First of all, the button should be inside it’s own row. Right now you just have an orphaned col-xs-12 which violates the Bootstrap standards. So doing that will separate them better. After that you need some margin between them

<div class="row">
<div class="col-md-12">
  <h2>test</h2>
  <div class="headline-date">Tuesday
    <button type="button" class="btn btn-primary pull-right">
      <i class="fa fa-user-plus " aria-hidden="true"></i>Call
    </button>
  </div>
</div>
</div>
<div class="row">
  <div class="col-md-12 ">
    <!-- members table -->
    <div class="col-md-8">
      1
    </div>

    <div class="col-md-4">
      2
    </div>

  </div>
</div>

And styling:

.col-md-8 {
  border: 1px solid red;
}

.col-md-4 {
  border: 1px solid blue;
}

.headline-date { 
  margin-bottom: 30px;
}

Example: https://jsfiddle.net/y9mznrna/1/