And many more, whose names on Earth are dark, But whose transmitted effluence cannot die So long as fire outlives the parent spark, Rose, rob'd in dazzling immortality. "Thou art become as one of us," they cry, "It was for thee yon kingless sphere has long Swung blind in unascended majesty, Silent alone amid a Heaven of Song. Assume thy winged throne, thou Vesper of our throng!"
There are others, the speaker says, whose influence ("transmitted effluence") lives on after their death.
They were merely sparks in a bigger fire of talent and insight, and the fire hasn't gone out. That's definitely another metaphor.
These visitors rise up from the grave, dressed ("rob'd) in "dazzling immortality." They are excited to welcome Adonais-Keats into the afterlife. He is one of them now, and they've been waiting to crown him king.
They ask him to assume the throne, which has stood empty amidst "a Heaven of Song."
The speaker is again using as a metaphor for poetry. There have been many poets in heaven, but none fit to sit on the throne until Adonais-Keats.
Shelley's basically saying here that Keats is the best poet of all time—wowza. He even calls the youth "Vesper," which is another way of saying Venus, the evening star. It burns the brightest, just like Keats.