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 because the German language has different expressions here:

– “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.”