In the short story THE MASQUE OF THE RED DEATH, what does Poe use to describe the guests at the party?

I'm going to assume you mean what literary devices he uses in his description. There are several: metaphor, imagery, parallelism, and polysyndeton. Each of these serves a unique function, adding to the overall mood of wonder and increasing the suspense of the story.

A metaphor is the direct comparison of something to something else unlike it. Unlike a simile, a metaphor does not use "like" or "as" in its comparison. One metaphor Poe employs is that of the partygoers as dreams. He describes them as having "writhed in and about, taking hue from the rooms, and causing the wild music of the orchestra to seem as the echo of their steps." When the clock sounds the party stops, but when the chimes end, Poe describes how the dreams "live" again.

Imagery is language that appeals to the 5 senses. Poe uses this extensively in his stories. For example, read this excerpt about the effect of the clock on the party.

The dreams are stiff-frozen as they stand. But the echoes of the chime die away—they have endured but an instant—and a light, half-subdued laughter floats after them as they depart.

The phrase "stiff-frozen" is touch imagery. There is also a great deal of sound imagery in the words "echoes" and "light, half-subdued laughter." Even the image of of the laughter floating appeals to our senses.

The last two devices, parallelism and polysyndeton, both have to do with the sentence structure. Parallelism involves phrases or clauses that are grammatically similar, but don't contain the same words. For example, "There was much of the beautiful, much of the wanton, much of the bizarre" describes the variety in the guests' costumes. Notice the phrases that begin with "much of." Polysyndeton is a fancy word for "a lot of conjunctions." Essentially, you use it anytime you repeat a conjunction several times. In the descriptions of the guests, Poe says "There were much glare and glitter and piquancy and phantasm." The repetition of "and" allows each idea to flow into the next, and gives the reader an impression of a flurry of action.