Questions & Answers about Nuestro grupo está muy unido.
Nosotros means we (a subject pronoun: we are, we go, we speak).
Nuestro means our (a possessive adjective: our group, our house).
In Nuestro grupo está muy unido, you need a word that shows possession of grupo (whose group?), so you use nuestro = our.
If you used nosotros, the sentence would be ungrammatical because you’d be putting a subject pronoun in front of a noun.
In Spanish, when you use a possessive adjective like mi, tu, su, nuestro, you normally do not add a definite article:
- Mi casa (not la mi casa) – my house
- Nuestro grupo (not el nuestro grupo) – our group
The possessive already gives the specificity that the article would give, so you don’t say el nuestro grupo in standard Spanish.