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