function onChange(control, oldValue, newValue, isLoading, isTemplate) {
	if (isLoading || newValue === '') {
		return;
	}
	
	var gr = new GlideRecord('sys_user_grmember');
	gr.addQuery('group', newValue);
	gr.query();
	if (gr.next()) {
		g_form.setValue('assigned_to', gr.user);
	}
	
	if (newValue == '') {
		g_form.clearValue('assigned_to');
	}
	
}

Try this

Hi Viktor, thanks for your script, it managed to populate the group member in "Assigned to" field once select the "assignment group". However, if clear the "assignment group" field, the "assigned to" value is not clear.

Please advise thanks in advance.  

Try this

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
if (isLoading) {
return;
}

var gr = new GlideRecord('sys_user_grmember');
gr.addQuery('group', newValue);
gr.query();
if (gr.next()) {
g_form.setValue('assigned_to', gr.user);
}

if (newValue == '') {
alert(newValue);
g_form.setValue('assigned_to', '');
}

}

This works perfect, thank you Shishir!