There are many transfer companies that run between Salzburg, Munich and Saalbach-Hinterglem.
Two that we find to be good value and reliable are:
Transferservice.at is our favorite (and cheapest) transfer company operating between Salzburg airport and Hinterglemm and Saalbach. They also do transfers from Munich to Hinterglemm. Make sure you book well in advance as they can get very busy at peak periods!
Holiday-shuttle.at are a local taxi and transfer company based in Saalbach. They operate between all close-by major airports and run a very good scheduled service between Salzburg and Saalbach-Hinterglemm.