Since souls are, by definition, human, I don't know if they can be personified. But that's beside the point, I guess.
In this poem, the speaker is saying that the soul is acting like a human being. She (the soul) is selecting who she wants as her friends (or perhaps as her lovers).
The poem says that the soul is making her choices based on what she thinks. She doesn't care if the person asking to be chosen is an emperor or what -- only those that measure up to what she expects will be chosen.
The soul is credited with being determined and confident in her choices. She is also given credit for being picky -- not just taking any old person.
Emily Dickinson personifies the idea of 'the soul' in several places in her poem 'The Soul Selects Her Own Society' if that is possible as a 'soul' is an ethereal ephemeral thing. Indeed the idea that humans have a soul at all is not accepted by every person. Dickinson however is clearly open to the idea and is intrigued to explore it further in her poem. Firstly, look in the title - so often overlooked in poetry. She refers to the soul being capapble of making choices - of having free will. She also refers to the idea of gender which is interesting as other ethereal entities such as angels are gender-less. So, we begin to get the picture that she may be talking about a particular soul - her own. The poet then gives the soul the ability to 'make things happen' (closing doors, albeit virtual ones.) Another action word or verb is 'notes' - this soul is capable of making observations and recording them. The idea of selection is clearly important to Dickinson, and to 'the soul' - she chooses to use another word 'choose.'
In two instances, the poet actually describes the soul performing physical acts:
a) "Then [the soul] shuts the door";
b) "Then close the valves of her attention / Like stone."
Philosophers have disagreed about the nature of the soul, but no-one has ever suggested that the soul has hands with which it can shut doors and close valves! Obviously, Emily Dickenson is using personification in the two above-mentioned phrases.
Emily Dickinson was known for being a hermit as well as a great poet. She rarely left her house and never published her work in her lifetime. This poem is about her tendency to introversion and her inability to interact with the larger world.
The soul stands in for the whole person. In fact, the soul is so separated from the speaker that it takes on its own personality, becoming regal and more powerful than the whole person. The soul chooses a friend and rejects all others, closing "the valves of her attention" and sealing herself off from others.
We get the feeling that Emily wanted to be more involved and extroverted, but something in her--a phobia, perhaps, or mental illness--would not allow it.