Someone might ask: why the heck do I need that? And yes, I never thought that I will need it but I learned that in SCOM 2007 a group is not always a group.

Jakub Oleksy helped me to understand and solve my problem and I am happy to share this information with you.

The problem is, that groups created by the web application wizard can not be tweaked using the UI in SCOM 2007. It's not possible to create a dynamic membership rule. It's also not possible to assign a subgroup to such a group. So having a lot of servers and a lot of web applications it's not really fun to pick every single server in every single web application.

Here my workaround:

• Create a web application using the wizard
  A group named after your web application with the suffix "watcher computers group" will be created
• Create your own computer group using the UI and define a dynamic membership rule
• Configure the script (the 3 lines at the beginning) and test (DO NOT RUN THIS UNTESTED IN PRODUCTION ENVIRONMENT!)

This is just a proof-of-concept script to show how you can do stuff like this. According to Jakub there may be a fix in SP1 which enables you to utilize the group created by the web application wizard in a way to either create a dynamic membership rule or attaching a subgroup, or both.

Disclaimer:
USE THIS AT YOUR OWN RISK. I AM NOT RESPONSIBLE FOR ANY DAMAGE THIS SCRIPT MAY CAUSE. THAT SAID, HAVE FUN!

Source Code