The grouping doesn't quite work like that, if you change your json to something like this:
[
{"ID":"1", "TIPIS":"GroupName1", "DESC":"name"},
{"ID":"2", "TIPIS":"GroupName1", "DESC":"name1"},
{"ID":"3", "TIPIS":"GroupName2", "DESC":"name2"},
{"ID":"4", "TIPIS":"GroupName1", "DESC":"name3"},
]
Then you'll get the grouping the way you want
jsFiddle: http://jsfiddle.net/rtCP3/182/