Poland chose not only to join the EU but also to become part of the Schengen Zone which means free travel for European citizens, with no visa requirements.
EU citizens, even from outside the Schengen Area, can enter Poland and seek work without needing a visa.
There are some countries which Poland has agreed to waive the visa on for short stays of up to 90 days. These include Japan, Australia, Canada and the US. However, if they want to remain within Poland for longer, they will also have to obtain a visa. If this is for business, they may need further supporting evidence, like a letter from a Polish employer.