<script src="https://code.jquery.com/jquery-3.3.1.min.js"></script>
<script src="https://vk.com/js/api/openapi.js?159" type="text/javascript"></script>
<script type="text/javascript">
$(function(){
  // init your application
  VK.init({
    apiId: 6720736
  });
  // click to 'Акцио!'
  $('.groupBtn').on('click', function(e) {
    e.preventDefault();

    $('.result').hide();

    const regex = /^(.*?\/)+(.*)$/;
    const str = $('.groupInput').val();

    let m;
    let groupId;

    if ((m = regex.exec(str)) !== null) {
      // the result can be accessed through the `m`-variable.
      const strId = m[2][0] + m[2][1];
      groupId = (strId == 'id') ? m[2].split('id')[1] : m[2];
      // request to callback vk.api
      VK.Api.call('groups.getById', {access_token: '137ef0e7137ef0e7137ef0e7ca13187c071137e137ef0e748bddfc5c5ae612adf3b5243', group_id: groupId, fields: 'members_count', v: 5.85}, function(r) {
        if(r.response) {
          $('.groupName').text(r.response[0].name);
          $('.groupAvatar').text(r.response[0].photo_200);
          $('.groupMembers').text(r.response[0].members_count);
          $('.result').show();
        }
      });
    } else {
      // error display
    }
  });
});
</script>
<input type="text" class="groupInput" placeholder="https://vk.com/testpool"><button class="groupBtn">Акцио!</button>
<div class="result" style="display: none;">
  <ul>
  	<li><b>Название</b>: <span class="groupName"></span></li>
  	<li><b>Аватар</b>: <span class="groupAvatar"></span></li>
  	<li><b>Кол-во участников</b>: <span class="groupMembers"></span></li>
  </ul>
</div>