Tryit Editor v1.8

Source Code:
<!DOCTYPE html> 
<html> 
<body> 
<button onclick="getMedGroup()" type="button">Get media group for videos</button>
<button onclick="setMedGroup()" type="button">Set media group for videos</button>
<br> 
<video id="video1" controls="controls">
  <source src="mov_bbb.mp4" type="video/mp4">
  <source src="mov_bbb.ogg" type="video/ogg">
  Your browser does not support HTML5 video.
</video>
<video id="video2" controls="controls">
  <source src="mov_bbb.mp4" type="video/mp4">
  <source src="mov_bbb.ogg" type="video/ogg">
  Your browser does not support HTML5 video.
</video>

<script>
myVid1=document.getElementById("video1");
myVid2=document.getElementById("video2");
function getMedGroup()
  { 
  alert("Video 1 media group: " + myVid1.mediaGroup +
 ". Video 2 media group: " + myVid2.mediaGroup);
  } 
function setMedGroup()
  { 
  myVid1.mediaGroup="test";
  myVid2.mediaGroup="test";
  } 

</script> 

<p>Video courtesy of <a href="http://www.bigbuckbunny.org/" target="_blank">Big Buck Bunny</a>.</p>

</body> 
</html>
Result:
W3Schools.com - Try it yourself