Usually a person who is at home takes the lift to go out and if he/she is out they take the lift to go back home.
So usually you can take the lift either to go to your own floor or to go to the ground floor.
This is something common to everyone living there but if, each person lives on a different floor, the ground floor is the same for everyone.
This means that finding the lift on the ground floor has a probability of 1 out 2.
In reality, this probability can depend on different factors. For instance, it is more likely to take a lift that goes up and not down, or it is likely that people can take the lift to move inside the building without going to the ground floor (e.g. to go to an office, to meet a neighbour, etc.).
