us is a pronoun used to refer to someone or something that consists of the speaker (first person) and at least one other person (second or third person). It can either be a subject or an object.
Us is used for both singular and plural nouns. When used with singular nouns, us is typically referring to the speaker and a group of people around the speaker or even just one person. When used with plural nouns, us is typically referring to the group of people as a whole.
1. Us girls are going to the mall later.
2. We need to discuss this between us.
3. Can you help us out with the yard work?
When using us as the subject of the sentence, be sure to use a plural verb. Additionally, it is important to note that us is never used to refer to just one person. This would need to be replaced with either I or me depending on the context.