Please see below email from Groundspeak. I highlighted the relevant part. Since I was actually there, I do know who found the cache that day.
Cheers and happy climbing
Thanks for updating the cache description. Technically, the guidelines don't specify that you must mark the log book yourself.
The guideline states: "Physical geocaches can be logged online as "Found" once the physical log has been signed. " It doesn't specify if the user has to log it themselves.
Thus yes, if a group of cachers find a cache and a user signs the names for all, this is allowed.
Your text on the cache descriptions is fine as it is.
This is more on the honor system. Some cachers may be ethically ok with claiming the find if they see the cache and someone else logs their name while in a group caching. Others aren't.
We aren't the logging police so we don't govern behavior to this extent. It takes some of the fun out of geocaching if the cache owner is deleting logs over this miniscule issue and may not be as much fun for cache owners either.
If it seems the cacher was not at the location and found the cache, it would be your responsibility to determine whether they can claim the find.
I hope this helps clarify. Please let me know if you have any further questions.
Community Support, Groundspeak
And this is the referred description:
So in compliance with the guidelines somebody has to put a mark into the logbook. It can be a sticker, a stamp, a signature or something else. If your buddy and not you puts the mark in the logbook, that's fine as well. However it doesn't count when you are not at the cache, which is up in the tree.