A better question is whether you have any guest passes you could use for one of your colleagues on your Qantas account? If you do, then it's a simple matter of transferring one to them and they can then apply it to their Qantas flight.
In terms of lounge access policies it will of course depend on what airline you are flying and what airline they are flying. If everyone is on a flight operated by Qantas, JetStar or a OneWorld carrier like American then there shouldn't be any issues. If on the other hand, you are flying Emirates, either marketed by Emirates or Qantas, then your colleagues must be on the same flight for access. I think it should go without saying, but if the flight isn't operated by Qantas, JetStar, a OneWorld partner or Emirates, then your Qantas Platinum status is as useful as a wooden nickel.
Is it possible to guest people into separate lounges as a QFF Platinum? Absolutely. Is it advisable? I'm less sure. First, let's look at the facts. At Melbourne international the business class and first class lounge are located in different parts of the airport. The business lounge is located downstairs (affectionately called the dungeon) whereas the first class lounge is located on the top floor. In theory yes you can go to the business class lounge initially with one of your friends and guest them in there, maybe sit there for 5 or 10 minutes (and tell your colleague you'll meet them at the gate for boarding) then pretend that you are taking a call "Hey sorry, you're breaking up, let me leave the lounge as I reckon the reception is bad", and head out. You could then take your other colleague to the First lounge and guest them in there. Oh and in case you are wondering, it is perfectly reasonable to leave and re-enter the lounge. I've done it before many times not just with Qantas but other airlines too. Now obviously if you are going back and forth every 15 minutes that's going to raise suspicion.
There is nothing wrong about utilizing one or more lounges when travelling. Many of us can and do take advantage of that benefit by the way (including guesting others into different lounges). For instance, there was a fairly infamous post I made a little over a year ago where I was flying out of LAX and there was an hour long wait to use the shower at the Qantas Flounge. I ended up leaving the First class lounge and hustling my way through the Qantas business class lounge to secure a shower ahead of my QF flight back to Sydney that was boarding in 30 minutes. Indeed a whole hobby can be made out of sampling lounges (Google lounge hopping). For instance, at LAX, whilst the Qantas First lounge has the best dining options, for work areas the nearby American Flagship lounge is far superior.