The lounge dragon/dragoness will insist you add the lounge invite to the reader at the gate towards the lounge.
They treat the invite like a purchase these days, ie, you have to link it to use it, or they won't let you in.
My first visit as a QF PS using my single entry, as soon as I get to the top of the escalator of the MEL T1 lounge, I am already asked if i have linked the invite to the flight...
No I say, the lounge dragon then asks me to log into my QFF account, and in there, I am not sure what he did, but after I logged in, he tapped something, and then, I am good to enter.
Now, on my QFF account, there is a notation of entry used, into the lounge.
If you have QP membership, its probably different, or if you are QF SG, you can just scan your card, but if its an invite, you have to link it (invite) to a flight.
Edit: my trip MEL - ADL was on Adelaide Cup public holiday, MEL was too, I guess.
Best to link before getting to airport, otherwise you will be holding up the time of one of the lounge dragons/dragonesses, as they have to show you how to do it.
There might also be people waiting behind you to have it done too.