Done it a few times. 1) Someone I'd just flown with who was on the same next leg and in the same long weather delay as I was; 2) a fellow-FTer in ORD on the same MR to try his hand at the JAL desk for seating--was an in-and-out job since boarding was near; 3) an AC-members 2nd guest that was being disallowed (as per standard AC policy) in LHR on a long layover.
As a general rule though, my club benefits are mine and not offered to, or shared with, others.