Learn Croatian: Jučer su se sastali s njim. - They met up with him yesterday.

Because of the preposition s (with). This preposition requires the noun or pronoun following it to be in the instrumental case. The instrumental form of on (he) is njim. So, "with him" translates to s njim.

Why is it s njim here, but sometimes I see sa instead of s?

The preposition "with" is normally just s. We only add the "a" (making it sa) when the next word starts with a similar sound (like s, š, z, ž) to make it easier to pronounce, or with certain short pronouns. Since njim starts with an "nj" sound, the plain s is easy to say. Just remember that "with me" is always sa mnom as a special exception!

Why is there a se in this sentence?

The verb here is sastati se (to meet up / to get together). It is a reflexive verb in Croatian, which means it must always be accompanied by the reflexive pronoun se (oneself). You can think of it literally as "They met themselves with him."

Why are su and se placed right after Jučer?