Learn Portuguese (Portugal): Quem são elas?

são is the plural form of the verb ser (to be) for "they" or "you all". Because we are asking about elas (they), we have to use the matching plural verb são. You would only use é if you were asking about a single person, like in Quem é ela? (Who is she?).

Why does this sentence use elas instead of eles for "they"?

In Portuguese, "they" has two forms. elas is used only when the entire group you are pointing to consists of females (or grammatically feminine words). If the group is all males, or a mix of males and females, you must use eles instead.

Since we are asking about multiple people, does Quem (who) need to be made plural?

No! Quem is an invariable word, which means it never changes its spelling or form. You will use Quem exactly the same way whether you are asking about one person (Quem é?) or a hundred people (Quem são?).

Can I change the word order and ask Quem elas são?

Yes, you can. In Portuguese questions that start with a question word like Quem, it is very common and natural to put the subject pronoun at the very end of the sentence, as seen in Quem são elas?. However, Quem elas são? is also correct and perfectly understood.