IF the custom property is in the Application table, then you should be able to pick it from the list of variables (Insert Variable) and it will be called ${App_group}.
If the custom property is in the nodes table, then your field will be a little more complicated. My "didn't try this first" guess is:
$SQL{select nodes.App_group from nodes where nodes.nodeid = ${NodeID} }
In both cases, you would put that code into the MESSAGE tab of the alert action.
Let me know if you get stuck or if I missed something there.