Do any airlines allow you to bring a guest from economy to the business class section of the plane?

Do any airlines allow you to bring a guest from economy to the business class section of the plane? In particular to use the etihad bar/lounge on the A380?

Almost certainly not.  I guess if you were some REAL celebrity then MAYBE.  The only thing I have seen over many years of travling is if there is seating available, just before landing, crew might bring forward children (travelling in Business) of First Class passengers  to be with parents in First for arrival.

I was traveling on Qantas12 in first from LAX to SYD some three years ago when a well known Australian actor seated by behind me requested the flight service manager, who arrivied promtly.His words to the FSM were..my three sons are upstairs in premium economy and did not get there courtesy up grade..i add speaking like it was allways expected.He then stated because his sons could not come into the first class cabin..he wanted to meet them in the upstairs lounge.To my amazement the FSM said if he would like to proceed upstairs he would escort the sons to the lounge.Shortly there after i went upstairs myself...and it was party time for all, the now intoxicated actor,his three adult sons and a bunch of p/j pax getting autographs.A good thing he was saying no pictures,no pictures because it would have been better than his previous mug shots.I think it depends who you are..but sure its  QF policy.

And here I was thinking that the 3 sons were pre-teens or something, not fully grown adult party animals! I am sure that when the FSD realised his mistake, it was too late to backtrack!  What an awful trip for you.

You don't say what the request entailed. If you mean the Y passenger to be re-seated in a spare J seat for the duration, with J service, then I'm sure the answer would be NO. I recall on a fairly recent GF107 flight in PE, the obnoxious guy behind me jumped up as soon as allowed and asked the FA if he could 'visit' someone he knew in J class. The FA let him, but escorted him back to his PE seat about 90 minutes later, clearly having guzzled considerable better quality booze. I'm guessing his acquaintance in J was pleased to see him go, even if his fellow PE passengers were not altogether thrilled to see him back.