Learn Italian: Tutti si sono congratulati con lei.

While "everyone" is singular in English, the Italian equivalent tutti literally means "all" and is grammatically plural. Therefore, the auxiliary verb must be plural (sono), and the past participle must agree with it (congratulati).

Why do we use si sono instead of just hanno?

The verb for "to congratulate" in Italian is congratularsi. It is a reflexive (or pronominal) verb, which means it requires the reflexive pronoun si. In the passato prossimo, all reflexive verbs must use essere as their auxiliary verb, not avere.

In English we say "congratulated her", so why does Italian use the preposition con?

The verb congratularsi is always followed by con to introduce the person you are congratulating. It helps to think of the literal translation as "Everyone congratulated themselves with her."

Why is it con lei rather than using a standard object pronoun like la or le?