Hello Grouplessgroupie,
Alright, with the issue happening with SMS and iMessage group messages we'd recommend deleting the existing group message thread, and separated threads on the iPhone and Apple Watch. Create a new Message thread, and see if the issue persists. If the issue persists after creating a new group message thread we'd then recommend unpairing the Apple watch, and then paring the devices again.
To delete the message threads on either device you can open the Messages application, then swipe from right to left across the thread, then tap Delete.
For unpairing the device you can find those steps in the following article. When unpairing and erasing the device a backup will automatically be created for you to recover when paring the Apple Watch back to your device.
Unpair and erase your Apple Watch
Set up your Apple Watch
Let us know how that goes.
Have a great day!