Learn Polish: Nie ma ich w pracy. - They are not at work.

When saying someone is absent or not in a place, Polish uses the fixed phrase nie ma. Even though ma normally means "has", the phrase nie ma acts like "there is not" or "there is a lack of". Because of this, it stays in the singular form even when talking about multiple people.

Which word means "they" here, and why does it look different?

The word for "they" here is ich. Normally, "they" is oni or one. But the phrase nie ma always forces the missing person or thing into the Genitive case. The Genitive form of oni or one is ich.

Is pracy a special form of the word for work?

Yes, the basic dictionary word for work is praca. Because we are talking about a location using the preposition w (in/at), the word changes its ending to pracy to match the Locative case.

Could I translate the English sentence literally as Oni nie są w pracy?

While a Polish speaker would understand you, it sounds unnatural. To describe someone's absence from a particular place, you should always use the nie ma structure.