This is the type of thing where, within reason, CX or other OW airlines will not send your friends/colleagues away. If you were to bring 1 or 2 guests with you to J checkin, there is no problem and I'm sure most Sapphire members will have done this, even on a different PNR.

Bringing in 10 guests on the other hand, would likely result in being politely told that your guests are welcome to use the Y checkin instead.

Where the line is drawn, nobody knows for sure... but I'm sure the staff have some guidelines somewhere.