Learn Croatian: Tko su oni?

Because the subject is plural (oni meaning 'they'). su is the plural form of the verb 'to be' that goes with 'they'. You might remember je from asking about a single person, but here we need the plural form.

Does oni mean 'they' for any group of people?

It is used for a group of males or a mixed group of males and females. If you were pointing to a group made up entirely of females, you would use one instead, making the question Tko su one?.

Why is the word order Tko su oni and not Tko oni su?

Short verbs like su are called clitics, and they have strict placement rules in Croatian. They almost always insist on being in the second position of a sentence. The question word Tko takes the first spot, so su must come immediately after it.

Can we drop the pronoun and just ask Tko su?

Yes, you can, because the verb su already tells us the subject is 'they'. However, when asking 'Who are they?', speakers usually include oni for emphasis, especially if they are looking or pointing at the specific group.