How do you go about deciding whom to add who your circle?

I have had several people add me recently, and I still haven't decided whether or not I want to add some of them. Some of them I have added, and some I have decided that I definitely don't want to add. But there are a couple of people that I'm not sure about. What factors do you take into consideration when you're deciding this? What are some dealbreakers or red flags that tell you that you don't want to add someone?