Learn Italian: Sono lì. - They are there.

Yes, you remembered correctly! In Italian, the verb essere (to be) uses the exact same word, sono, for both 'I am' (io sono) and 'they are' (loro sono). You usually figure out which one it means from the context of the conversation.

Why is the word for 'they' missing? Could I say Loro sono lì?

Yes, you can absolutely say Loro sono lì. However, Italians usually drop subject pronouns like loro (they) because the verb and the context already show who is doing the action. You only really need to include loro if you want to emphasize who is there, like saying 'THEY are there.'

Why does lì have an accent mark?

The accent mark on lì (there) doesn't change how it sounds. It is used to tell it apart in writing from another short Italian word, li, which means 'them' and is used differently.