我从句法上区别一下：

■It is agreed that…

it是形式主语，that引导真正意义的主语从句，这个结构中的be agreed是被动结构，也就是说，agree是动词。例如：

It was agreed that another meeting was necessary.

大家一致认为有必要再开一次会。

= We agreed that another meeting was necessary.

■sb. be agreed that…

agreed在这个结构中是形容词，意为“一致同意的；意见一致的”，它可以用于be agreed on …/ be agreed that…等结构。例如：

Are we all agreed on that.

在那一点上我们意见一致吗？

We are all agreed on leaving the room at once.

We are all agreed that the proposal is a good one.

我们都认为这个建议很好。

至于是什么从句，张道真教授《实用英语语法》中指出，类似这种从句在概念上接近宾语从句，而在结构上接近状语从句。呵呵，如果只是就实用角度来说，我觉得会用就好啊，如果是教师身份可以深入了解一下。