While it's always tempting to read autobiographical elements into a piece of written work, sometimes it can lead to a distorted understanding. So it's always best to proceed with caution in such cases. That said, there are few more autobiographical poets in the English language than Emily Dickinson, who put so many elements of her own life into her extraordinary body of work.
"The Soul Selects Her Own Society" is a prime example of this. Dickinson was something of a recluse, who consciously shut herself away from the outside world. Numerous anecdotes abound of her social isolation. As well as speaking to visitors through doors, she'd also lower baskets of treats to children from her window. She even listened to her father's funeral from the privacy of her bedroom.
It seems fair to say, then, that this poem, which affirms the sacred right of the individual to choose his or her own companions, is an accurate reflection of its author's eccentric personality. The speaker is so self-enclosed that she'd refuse to admit anyone from outside her charmed circle of friends into her presence. Even if a mighty Emperor should turn up, he would not gain admittance. He'd have to make do with kneeling down on a mat.
From what we know of Emily Dickinson's life and personality, this is by no means an exaggerated sentiment. If a real-life Emperor had rocked up at her house in Amherst, then it's almost certain that he, too, would not have been allowed inside.