Yo de acuerdo
Is this correct?
I agree = Estoy de acuerdo
You agree ( Singular)= Tú estás de acuerdo
He, she ,usted agree = él/ ella/ usted está de acuerdo
We agree = Nosotros estamos de acuerdo
You agree (Plural) = Vosotros estáis de acuerdo
They agree. = Ellos/ellas/ustedes están de acuerdo
3 Answers
I have heard this used often by advanced speakers, so I am pretty sure it is correct. A completely literal translation would be 'I am in agreement'; same thing, really.

Ustedes is not They argee.
Ustedes están de acuerdo.= You (plural) agree. The same as vosotros.
But ustedes is formal, while vosotros is informal, right?
In general, yes.
Ustedes is 2nd person, plural, formal, while vosotros is 2nd person, plural, informal.
The twist is that in Latin America the 2nd person, informal, plural (vosotros) is not used, So if you were speaking to a friend using tú and another friend joined him, you would use 2nd person, plural, formal, even though they are both friends. (In other words there is no informal way to talk to two people. You speak to them formally. (or, at least, you use the formal conjugations).