First of all, append a checkbox element ahead of the group title.
groupFormatter:function(value, rows){
var s = value + ' - ' + rows.length + ' Item(s)';
s = '<input type="checkbox" onclick="checkGroup(\''+value+'\',event)">'+s;
return s;
}
Define the 'checkGroup' function to check the group rows.
function checkGroup(value,event){
value = $.trim(value);
var dg = $('#dg');
var groups = dg.datagrid('groups');
var group = null;
for(var i=0; i<groups.length; i++){
if (groups[i].value == value){
group = groups[i];
break;
}
}
var checked = $(event.target).is(':checked');
for(var i=group.startIndex; i<group.startIndex+group.rows.length; i++){
dg.datagrid(checked?'checkRow':'uncheckRow',i)
}
}