Copy-to-clipboard button - https://codepen.io/richarddewit/pen/jQZoyb

copy-contents.js

var copyBtn = document.querySelector('.js-copy-button');
var originalContent = copyBtn.innerHTML;

copyBtn.addEventListener('click', function(event) {
  var copyContents = document.querySelector('.js-copy-contents');
  // Clear selection first
  window.getSelection().removeAllRanges();

  // Select the contents
  var range = document.createRange();
  range.selectNode(copyContents);
  window.getSelection().addRange(range);

  try {
    // Do the copy
    document.execCommand('copy');

    // Flash success message inside button
    copyBtn.disabled = true;
    copyBtn.innerHTML = 'Copied!';

    setTimeout(function() {
      // And return to original state
      copyBtn.disabled = false;
      copyBtn.innerHTML = originalContent;
    }, 1500);
  } catch(err) {
    console.log('Oops, unable to copy');
  }

  // Remove the selections - NOTE: Should use
  // removeRange(range) when it is supported
  window.getSelection().removeAllRanges();
});

invitation.html

<div class="well js-copy-contents">
  https://example.com/invitation?key=7a8f6s78afs8a7ft678t22nin22j
</div>

<button class="btn btn-primary js-copy-button">
  Copy invitation link
</button>

CodePen: https://codepen.io/richarddewit/pen/jQZoyb