The best way to go about using the log level alert for a group is to take advantage of the Administration Groups feature in EM. Essentially, you can then assign a WLS metric template to the entire group and setup notifications appropriately. You can look up Administration Groups in the online help in the product.
Are you using EM 12c R2? If so, you will find a wild card capable feature right in the list of WebLogic Server metric thresholds that can alert on any string in the log. The feature is new in R2, so you wouldn't see it in the list of metrics within the EM 12c BP1 release. That is the metric that you should alert on via the Administration Group.