I'd just add that a group can agree "on" a decision, even if an individual cannot. However, the sense in that case is that the group has reached a decision, not that the group agrees with a decision already reached beforehand.
I don't think so, but maybe you have an example that would help. Every usage of "agree on" that I can think of off-hand requires more than one person to do the agreeing. For example, I don't think an individual can agree on a decision. With whom would the individual be agreeing? It takes a group to agree on a decision, or on a course of action, or on what to have for dinner.