The correct choice is us.
Along with the noun Naomi, the pronoun us is an appositive. Since appositives are nouns or pronouns placed after another noun or pronoun in order to identify, rename, or explain it, Naomi and us are placed after the noun juniors, and juniors, identifying who the juniors include; namely, Naomi and us. And, like juniors, the appositives (Naomi and us) act also as Direct Objects. Therefore, it is necessary to use the Objective Case pronoun us for this Direct Object appositive.
One way to verify that you have selected the correct case of the pronoun used as an appositive is to read/write the sentence without the Direct Object and the other part of the appositive, Naomi. Doing this will allow you to see and hear that you have chosen the appropriate pronoun:
e.g. Mr. G recognized (we, us) at the last assembly. [Here you will know that you need the Objective Case us for the Direct Object of the verb recognized.