I'm not sure about the danger of being arrested as I don't live near there and don't know the laws or how airsoft is seen there, might be different than here.
The BB's are not dangerous to the back at all, most likely will not break skin, but if someone was shot in the face which isn't very uncommon in airsoft, if you don't have eye protection, you can easily blind someone. Any goggles that will cover your eyes will be better than nothing, but goggles with paintball or ballistic ratings would be the best choice.
I agree with you, you should stear clear. If it's not an organized event, anything can happen, and authorities could be called and get involved. Not to mention the safety issues, not a smart plan. Tell your friends that they are stupid and are risking a lot!