Thanks, tschoof!
Group chats are a great idea!
What follows is a technical explanation why they aren't possible right now (sorry!):
The problem is: iMessage's AppleScript support isn't documented at all, but after some Google research, it looks like iMessage's handle for group chats gets generated automatically (something like "chat354574422665" <- random number), while the handle for a single user is his phone number or email address. I hoped that it's just some kind of array ("012345,678910" or "012345+5678910"), but it's not. I extracted the group chat number from iMessage's SQL database and tried it as the new recipient, but that didn't work either. Although the text will be linked to the right group chat iMessage reports that "chat354574422665" isn't a valid iMessage address.
I really like the idea of group chats and I'm watching out for a workaround. If anybody has any idea how to solve this, please contact me.