How can I set the maximum length of 6 and minimum length of 6 in a textbox?

You could do it with JavaScript with something like this:

<input onblur="checkLength(this)" id="groupidtext" type="text" style="width: 100px;" maxlength="6" />
<!-- Or use event onkeyup instead if you want to check every character strike -->

function checkLength(el) {
  if (el.value.length != 6) {
    alert("length must be exactly 6 characters")
  }
}

Note this would work in older browsers which don't support HTML 5, but it relies on the user having JavaScript switched on.

You can use the HTML5 pattern attribute or use JavaScript.

The pattern could look for example like this:

<input id="groupidtext" type="text" pattern="(.){6,6}" style="width: 100px;" maxlength="6" />

But the pattern attribute will only work with HTML5 browsers. For old browsers you'll need JavaScript.

As suggested in the comments to add, this will only work as soon as a form is about to be submitted. If this input is not in a form and you need validation as a user types, use JavaScript.