Shinigami enter and leave Soul Society by unlocking the Senkaimon using a Hell Butterfly. Pluses, on the other hand, enter Soul Society through the ritual of Soul Burial. Spirits can be born in Soul Society as much as in the living world, with the Shinigami acting to balance the souls.
Intruders can pass through the Senkaimon in the same manner, but only in soul form. This allows spiritual beings, such as Ichigo in his Shinigami form, to enter without permission. Humans can also use such a gate through the use of Reishihenkanki (spirit exchangers), which convert solid matter into Reishi's. While this method uses the same gate, the lack of a Hell Butterfly forces intruders to pass through the Dangai, a bordering dimension between the human world and Soul Society (not to be confused with Hueco Mundo). This is much more dangerous to do, as souls that cannot pass through before the gate to Soul Society closes are trapped there forever.
Finally, Hollows can also directly rip a hole to Soul Society from Hueco Mundo, or in the case of Arrancar, using their ability, Garganta.
When people die they are assigned a number to a part of Soul Society according to when they died. Spirits can be born in Soul Society as much as in the living world with the Shinigami acting to balance the souls.
