Learn French: Quelques bénévoles sont déjà ici. - A few volunteers are already here.

Quelques specifically means a few or a small number of, which emphasizes that there are not many volunteers yet. If you used des, it would just mean some volunteers without that emphasis on a small quantity.

Does quelques change depending on whether the noun is masculine or feminine?

No! Quelques always ends in -es because it is always plural, and it has the exact same form for both masculine and feminine nouns.

Is bénévoles a masculine or feminine noun here?

It could be a mix of both! The singular word is un bénévole (a male volunteer) or une bénévole (a female volunteer). Since it already ends in an -e, the base spelling does not change for gender. Because it is plural here, we just add an -s.

Why does déjà go in the middle of the sentence?

In French, short adverbs like déjà (already) are usually placed immediately after the conjugated verb. That is why it goes right after sont (are) and before ici (here).