I just went through this with Solarwinds support.
According to them there is no way to combine different types of "Property to Monitor" in a single alert. For example, if under, "Trigger Alert when all of the following apply" you add "Volume Percent Available is less than or equal to 10" you can also add and "Group Name is equal to MyGroup." If you do, you will get the dreaded "Field not specified or invalid" error.
Support had me create a Custom Property and then add all of the volumes I wanted to alert on (all the fixed disks for servers in that group). This worked but seems pretty cumbersome.
Right now I literally simply added each of the 36 nodes I wanted to monitor to the trigger condition.
I'd love to hear how other people did this.
Pjc