Looking for a solution to add a data attribute to a K2 Category item list.

want to look like:

<div class="itemContainer itemContainerLast" data-group="[1,2,3]" >

You need to add these classes directly to your template, this post will help you locate the files you need to change .
getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates