I know a companion on a different PNR can be upgraded with us (although issues may arise at checkin). The website doesn't answer my other questions, and only says the following:

Unlimited ability to upgrade one person into First Class if traveling with you*

* Subject to availability. Certain restrictions apply.

If I am in paid F, and want to upgrade a companion on a different PNR, I'm sure it has to be in U space, but what about fare restrictions? Does it have to be H and above to qualify in advance outside of 3 days? Are the "certain restrictions apply" viewable anywhere? Any problem with a different person in each direction? (to treat different colleagues each way).

Thanks