Emily could be writing about herself as a poet who every day will “grow unnoticed” and she writes only because she must, not for fame or “applause”. Yet she also describes “fellowship” in contrast to the lonely work of building a mountain but who is it she desires “fellowship” with? Is she suggesting that the light of the sun (inspiration) spends each night with her in dreams?
I would imagine Emily never actually saw the mountains, at least not the kind she’s describing in this poem. However, not having first-hand experience is no impediment to the creative mind, and I think that’s partially what she getting at here, especially with the final image of the poem in which the mountains turn “golden” and then “night” comes. This image is important because the light of the sun which had been illuminating the “Faces” of the “Mountains” ends its day tucked in with the mountains, as if the light of the “Sun” which reveals all that which is visible to the naked eye, shines differently at night and illuminates the unseen world. If you think about how dreams work, especially the fact that you can see even though your eyes are closed, where is this light source coming from if not the “Sun” who has nestled itself in the mountain range of your dreams?
Thus Emily, who does not often leave her home, is still able to travel across the universe every night and then reports back her findings the next day in her poetry. She is, like the geologist who studies how “Mountains – grow”, she records the growth of her imagination and what is revealed there in her own notes, and she does so without asking for any recognition or help. Her only companion is her imagination and that seems to be plenty of “fellowship” for her.