Hi,

I don't know if it's my mistake or if it's a grammatical error. To me it should be: On nous a envoyé.

If it's "envoyés", could you explain why?

on a envoyé chercher qui (= complément d'objet direct / COD) −> réponse : nous

« on nous a envoyés » car le COD est placé avant l'auxiliaire avoir.

It's the trickiest rules among french grammar rules.
When the COD is located before the auxiliary « avoir » in a sentence, the participle have to be agreed with it (but in any case, we never have to agree the participle with the subject with the auxiliary « avoir »).

Thank you very much for the explanation.