W3Schools Tryit Editor

<!DOCTYPE html> 
<html> 
<body>

<video id="myVideo1" width="320" height="240" controls>
  <source src="movie.mp4" type="video/mp4">
  <source src="movie.ogg" type="video/ogg">
  Your browser does not support the video tag.
</video>

<video id="myVideo2" width="320" height="240" controls>
  <source src="movie.mp4" type="video/mp4">
  <source src="movie.ogg" type="video/ogg">
  Your browser does not support the video tag.
</video><br>

<button onclick="setMedGroup()" type="button">Set media group for videos</button>
<button onclick="getMedGroup()" type="button">Get media group for videos</button>

function setMedGroup() { 
  x.mediaGroup = "test";
  y.mediaGroup = "test";
}

function getMedGroup() { 
  alert("Video 1 media group: " + x.mediaGroup +
  ". Video 2 media group: " + y.mediaGroup);
} 
</script>

</body>

</html>