Recommendation is to use the Custom Property Manager to create a Custom Property at the Node level and assign values to that Custom Property in order to group the nodes by Custom Property value instead of by Node Group.
How we do this at our company is to create a Node "Custom Property" and then put a value for that custom property into the nodes you want to trigger alerts for. So, for example, you could create a Custom Property called "NodeGroup" and then populate the values of the NodeGroup property for the nodes. Then, in the Alert Trigger, you would put in the condition of Custom Property NodeGroup = "YourValue".
All of our Advanced Alerts are triggered off of Node Custom Properties.
In SolarWinds, the Node Groups are only for Status purposes and you will find you are better off using Custom Properties instead to group your nodes. This will make it easier to trigger other things in SolarWinds as well (like View limitations, etc.) if you use Custom Properties to group the nodes.