.popup {
  display: none;
  position: fixed;
  z-index: 999;
  left: 0; top: 0;
  width: 100%; height: 100%;
  background: rgba(0,0,0,0.5);
}
.popup-content {
  background: #fff;
  padding: 20px;
  margin: 10% auto;
  width: 300px;
  border-radius: 8px;
  text-align: center;
  position: relative;
}
#close-popup {
  position: absolute;
  right: 10px;
  top: 5px;
  cursor: pointer;
  font-size: 20px;
}
.join-btn, .submit-btn {
  background: #0073aa;
  color: #fff;
  border: none;
  padding: 10px 15px;
  margin-top: 10px;
  border-radius: 5px;
  cursor: pointer;
}