Formal:
* Concordant - meaning in agreement or harmony
* Harmonious - meaning characterized by agreement and peace
* Consonant - meaning in agreement or harmony
* Unanimous - meaning characterized by complete agreement
* Conciliatory - meaning tending to pacify or reconcile
Informal:
* Peaceful - meaning free from disturbance or violence
* Friendly - meaning characterized by goodwill and affection
* United - meaning joined together as one
* Cooperative - meaning working together for a common purpose
* Agreeable - meaning pleasing or acceptable
The best adjective to use will depend on the specific context in which you are using it.