As has been noted, some airline fora (QF in my case) have threads devoted to posting details of when one is lounging and whether one can 'guest' someone.

On that basis I wouldn't have a problem with someone requesting I guest (host?) them into the lounge (be it J or F) - it's my choice to post those details of when I'll be there and whether I have a guest spot available.

I'd be happy, in certain circumstances, to request same, but would be never just ask a total stranger 'out of the blue' (i.e. would need to 'know' the FT/AFF member involved)