Just like in French, Italian has two types of verbs in the past perfect:
1. verbs which take avere (the majority)
2. verbs which take essere (fewer than 20).
The verb "arrivare" is part of the second group which takes essere.
Unlike the verbs from the first group which take avere, verbs which take essere must also agree in number and gender. Therefore, because "noi" is plural, the past participle must be "arrivati", rather than "arrivato".
We arrived = Noi siamo arrivati