There are many “tours“ dedicated to tricking people, mostly tourists and new arrivals. Or else their schemes would not work, because all the locals have heard of them and they will not fall for them. Basically they take you to various bars that are always free, where you only pay for your drinks. But these “tours“ tell you that with their wristband you don´t have to pay the entry fee, which does not exist, and these “tours“ keep your money. After these bars they promise you entry into discos, most of the time they offer the biggest or most famous ones, like: Joy, Kapital, Velvet, Gabana, Teatro Barceló, etc. Then comes the worst part of all, because once you are with your group in front of the disco, the “guide“ will tell you that the place is overcrowded and you can´t get in, or that the group is late, and now you have to pay the entry fee (even though they said, don´t worry because you only pay for your drinks before), or maybe the Disco will not allow you to enter without a shirt ,and you are wearing a polo. There are many ways you could be tricked, another common example is that most of the bars will not ask for your ID at the beginning of the night, but the majority of Discos will always ask for it, and you may not have brought it to avoid losing it, and the promoter didn´t tell you anything because they don´t care (and it makes it easier for them), however, he won´t see you again (or very often) and your night will be ruined.
As you can see, there are many ways to be screwed over in this city so, some advice from somebody who has been cheated in many ways is: to go directly to the entry and pay your entry fee, find an official promoter (PR/RRPP) of the place, make a reservation on the internet , or find a company that is dedicated to these kinds of tours like the Erasmus Networks of the Universities (Erasmusmadrid, ESN, etc.), Pubcrawls that take groups of people from hotels/hostels, travel agencies, etc.
If you find yourself in this situation, please call the Police at 092, so you can get your money back, at least.