Change your markup slightly:
$(function() {
enable_cb();
$("#group1").click(enable_cb);
});
function enable_cb() {
if (this.checked) {
$("input.group1").removeAttr("disabled");
} else {
$("input.group1").attr("disabled", true);
}
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<form name="frmChkForm" id="frmChkForm">
<input type="checkbox" name="chkcc9" id="group1">Check Me <br>
<input type="checkbox" name="chk9[120]" class="group1"><br>
<input type="checkbox" name="chk9[140]" class="group1"><br>
<input type="checkbox" name="chk9[150]" class="group1"><br>
</form>
You can do this using attribute selectors without introducing the ID and classes but it’s slower and (imho) harder to read.