I can explain it for Republican Rome. A complex system of patronage existed in Rome and is the closest to a system of lobbying though it was very different. What happened was a mutually beneficial relationship would exist between a client and his patron.

The nut and bolt of it was that your importance in Rome was in part determined by how many clients you had. In return for the client giving the patron "dignatis" and helping the Patron whenever the patron asked for it the client got someone in power who would look after him. This would normally take the form of representing the client in court, or loaning money and supporting the clients attempts to run for office.

Source Cicero: Anthony Everrit