Disable checkbox array when radio button is not checked

Im trying to disable a checkbox array when the corresponding radio button is not checked. My code disables only the first element of the checkbox array and NOT the rest of the array. I might be doing something wrong but I cant seem to figure it out.

Any help would do.

Javascript :

<script>
  function radioDisable(){
    if(document.getElementById('rdGrp').checked){
      document.getElementById('txtUserID[]').disabled=false;
    }else{
      document.getElementById('txtUserID[]').disabled=true;
    }
  }
</script>

html :

<td>
  <input type="radio" name="rdAll" id="rdAll" value="all" onchange="radioDisable()"> All Users
  <br>
  <input type="radio" name="rdAll" id="rdGrp" value="group" onchange="radioDisable()"> Groups
  <br>
  <input type="checkbox" name="txtUserID[]" id="txtUserID[]" value="SysAd"> System Admin |
  <input type="checkbox" name="txtUserID[]" id="txtUserID[]" value="faculty"> Faculty |
  <input type="checkbox" name="txtUserID[]" id="txtUserID[]" value="student"> Student |
  <input type="checkbox" name="txtUserID[]" id="txtUserID[]" value="registrar"> Registrar |
  <input type="checkbox" name="txtUserID[]" id="txtUserID[]" value="adviser"> Adviser |
  <input type="checkbox" name="txtUserID[]" id="txtUserID[]" value="clerk"> Clerk | 
  <input type="checkbox" name="txtUserID[]" id="txtUserID[]" value="management"> Management
</td>