sie = she or they
Sie = you [formal; singular and plural]
ihr = you [informal; plural]
du = you [informal, singular]
PS: The above applies to personal pronouns (I, you, he/she/it ...) that are in the nominative case. The nominative case is used for the subject of a sentence as in the sentence in the excercise. In other cases, the forms of the personal pronouns change, e.g. "Sie" (you) becomes "Ihnen" (to you) in the dative case. Also, "ihr" is not only used as a personal pronoun meaning "you", but also as a possessive article meaning "her" (e.g. ihr Buch = her book).
"Can you imagine us" should be accepted too , because uns is both dativ and accusativ and while vorstellen have two ways to use : 1- Ich stelle mich vor (Akku : I introduce myself) 2- Ich stelle mir vor ( Dat : I imagine ) ....... or am I wrong ? or the Imagine way should just be used as a dativ reflexive ?
No, it's a separable (= trennbares) verb, e.g.: "Ich stelle mich vor". However, when a separable verb is used in the infinitive form as in this sentence, its two parts are not separated.
For more information, see: http://www.nthuleen.com/teach/grammar/sepinsepprefixexpl.html