It was agreed on that we should go together.
句中的 on 没有用错,不过可以省略。
如果是主动语态,that 从句前面是不用介词 on 的。如:
They agreed that we should go together. (不能有介词 on)
但在相应的被动语态中,则可以加上介词 on(但仍以省略 on 更为普遍):
That we should go together was agreed (on).
It was agreed (on) that we should go together.