Aside from being a VIP, there are generally there are 4 ways to access lounges.
1) Qualifying by virtue of flying in first/business class. Most airlines with first or business class provide lounge access for their passengers in those classes. Note some exceptions apply - eg Qantas domestic business class does not give lounge access except in some circumstances, nor United/American Airlines domestic first class, and not all airports may have a lounge.
2) By paid airline club membership - eg Qantas Club, Admirals Club, Koru Club etc. Usually have to be flying one of selected airlines same day to use the lounge. Again may not be available at all airports those airlines fly from.
3) By membership of non-airline club - eg Priority Pass, some high ranked credit cards.
4) By airline frequent flyer status. Many airlines provide lounge access to higher ranked status members of their frequent flyer program (FFP). As alliance benefits this may be extended when flying within the alliance (or may not - eg Lufthansa Miles & More frequent traveller status (star alliance silver) provides access to own lounges when flying LH/LO/OS/LX (Lufthansa / LOT / Austrian / Swiss) but not when flying other star alliance airlines).
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