English has similar intensive pronouns: myself, yourself, herself, etc. (these are also used as reflexive pronouns, but the intensive use is different). You could say You will clean the room. But if you wanted to emphasize that no one else should help clean the room you could say You yourself will clean the room or You will clean the room yourself. a tí has a similar emphasis in Spanish.
And another way to add this type of emphasis is to add mismo after the pronoun: ¡Hazlo tú misma! (You do it yourself!).
It's just the object form of "you" (second person singular pronoun). It's the same difference between "I" and "me"; you use "I" when the speaker is one acting and you use "me" when the speaker is the one being acted upon. Likewise, you use "tú" when the listener is the one acting and "ti" when the listener is the one being acted upon. E.g.: "Tú caíste sobre una manzana" versus "Un manzana cayó sobre ti".
Do our comments matter to the correctors at DuoLingo? Because "they are including you" is absolutely correct for "te incluyen". the present progressive in English is often not only an acceptable translation for the present simple in Spanish, but is also even the preferable translation!