I don't think you'll be able to get any alerts that would be able to split by agent groups. The only measures that consider agent groups around web services don't consider failures so it would need to include a business transaction which you would be able to split by method or endpoint and there is no splitting by agent groups for business transactions that I am aware of. I would say the closest you could get is an incident based on the failure rate of a BT split by web service. Also note that the failure rates will be based on the entire PurePath not just the web service call.
James
