What is the ruling on taking money in exchange for covering up those who bring girls into the building where I work as a security guard?

Your permission for unlawful mixing (of sexes) in the building and taking money for it is forbidden. It is a form of cooperating in sin and aggression. What is obligatory upon you is to repent, feel remorse, and resolve not to return to it, and to end the wrongdoing by changing it and informing the owner of the building and the official authorities about what is happening.