In order to properly express agreement in German it’s important to consider whether you’re agreeing TO DO something or agreeing WITH someone’s opinion.

As noted, the German language has different expressions for different types of agreement.

Ich bin einverstanden. I agree (to this action).

Ich bin deiner Meinung. I agree (with your opinion).

The noun “agreement” is Vereinbarung (f.) in the sense of a contractual agreement or a formal arrangement. To be “in agreement” one would have to use Übereinstimmung (f.).

Wir haben eine Vereinbarung über die Vertragskonditionen. We have an agreement concerning the contract conditions.

Wir sind in Übereinstimmung mit den Regeln. We are in agreement with the rules.