Possession of small amounts of drugs for personal use is indeed not a criminal offence in the Czech Republic (it is, however, still not fully legal; it’s an administrative offence, similar to a parking offence). But this is for personal use; the trafficking or sale of drugs is still a criminal offence (cite). So if you want to be on the safe side, the patrons of your orgy venue in Prague would have to bring their own supply of drugs, which I think would kind of defeat the purpose of the scheme.