I've had many cases where people would mistake group instances for normal instances and abide by normal instance rules where some things would be different to the group. A little pop up telling them that they're in a group instance and that they should abide by the group rules when they join would be a great way to Let people know that they should stick to the rules of the group.

Now merged with https://feedback.vrchat.com/feature-requests/p/1306-group-instance-welcome-message