I'm planning on going to Khalitea, Greece, but don't have too much money for hotels with my mates, so we got an idea to set up a tent or two on the beach, sleep there and go to the discos. Would this be legal? We are planning about 5 nights.
In Greece, as in many other European countries, wild camping with a tent or a caravan is prohibited according to a 1976 law.
The article goes on to investigate the difference between campervans and camping with a tent, but long story short, no, it's not legal, but you'll likely get away without a citation if you do it.
Which Kalithea are you referring to? Kalithea in Halkidiki, or Kalithea in Rhodes? Free camping is illegal in Greece but it is tolerated in remote beaches, away from camping sites, hotels and settlements. Choose a site preferred by local free campers.
Unfortunately there currently are no official camping sites in Rhodes. It may be possible to camp for a couple of nights in the remote beaches of the southern part of the island, but this is far from Kalithea where you say you want to be based.
So perhaps sharing an Airbnb apartment is the best value for money, or, of course, a flight+room package.