Short answer, "yes"
Medium answer: if that's in the nodes table, you can refer to it as ${node.APP_Group} - note that in the alert variable, the "nodes" table is called "node" instead.
If you have it in the application table then the answer is still "yes" BUT how you get there depends greatly on what type of alert trigger you are using - APM_Application, APM_Component, custom SQL, etc.
Can you give a screen shot of the trigger and tell us where the custom field is?