Last time I read the regulations fully it was my understanding if YOU don't have a valid tag it is illegal for you to fill someone else's tag because you legally can not be hunting without a valid tag for yourself. I think very few party hunting groups actually follow this law. I know a couple groups that guys buy doe tags specifically so they can hunt all 9 days and possibly fill someone else's buck tag in the group.
Another reason why I really dislike gun season. Which is funny because that's what got me into hunting to start with then I was 12.
Deer hunting traditionally was families and friends hunting together to get meat . Some people say you shouldn't be in the woods if you got your buck . I say if your Dad wants you to fill his tag while he's out there why not ? As long as it's a valid tag who cares who fills it.
We had those cool two-part tags at the time. I've proposed that we should go back to those tags and if you don't have a full back tag, you should not be allowed to be in the woods with a firearm that is (by the legal definition) usable for deer hunting.