Is your cat lonely and want to get another as a companion? Is someone giving you a cat because they no longer can take care of it? Have you become the neighborhood cat person?
You can definitely bring a new cat into your environment but make sure you do it correctly! Just remember you CANNOT force cat to be friends or even just friendly to one another. Though there are ways to help their transition, there is also no absolute way to make them buddies. Think on your cat’s personality and really consider if they are going to work.

Introduce them slowly, keep the new cat in a small room with their own litter box and food. This will allow the first cat to get used to their smell and sound. Cats are very territorial, so this is a good way to allow them to get used to one another.
Another good idea is to feed them at the same time but on opposite sides of a door. This will get them familiar with eating with another and they won’t try to steal from the other. They will smell and hear each other.
After a few days switch the blankets they use to sleep on or cat beds so they can smell each other more. Smell is very important cats and other animals in fact. To help the new cat feel more at home, switch them so that the new cat is able to explore the house while the original cat stays in the smaller room and smell the new cat’s stuff.
For more information check out the article the SPCA came out with about cat to cat introductions.