What are the similarities and differences, for example, between "they are agreed" vs "they've agreed." Also, is there a formal grammatical term for the construct "they are agreed" ?
http://www.usingenglish.com/forum/threads/96405-We-are-agreed claims no differences:
In agreement is simply a phrase that means having the same opinion. Agree, as you use it, is an intransitive verb meaning to be in accord or having the same opinion.
So, they are structurally different, but almost semantically the same. That is, they have the same meaning. Yes, consensus has almost the same meaning, but its used in formal contexts.
Per contra, this DOES claim a difference:
They "are" agreed is stative, they are in a state of agreement, and this doesn't necessarily mean that they spoke to each other and made the agreement, it might simply mean that they have all taken the same position independently. But when you say "they have agreed" it does imply that they have got together and agreed a position.
I'm contending against this latter argument. If they have all taken the same position independently, then haven't they agreed? Does it matter whether they acted alone or conferred?