Learn Croatian: Ljudi su tamo zajedno. - The people are there together.

The subject is Ljudi (people), which is plural. su is the plural form of the verb "to be" (are) that matches "they", whereas je is the singular form (is).

I learned that oni su means "they are". Why isn't oni used in this sentence?

Because the sentence already has a specific subject: Ljudi (the people). You would use the pronoun oni (they) if you didn't specify who you were talking about. If you just wanted to say "They are there together," you would say Oni su tamo zajedno.

Could we put su at the end and say Ljudi tamo zajedno su?

No, because su is a "clitic" (a short, unstressed word). In Croatian, clitics must take the second position in the sentence. Here, it sits perfectly in the second spot, right after the first word, Ljudi.

What is the singular form of Ljudi?

Ljudi means "people". Its singular form is completely different: čovjek, which means "man" or "person". This is very similar to English, where the plural of "person" is "people"!