The shareholders cannot get involved in negotiations. This is not just about the mandate. During the negotiations, and even at the end of them, you can intervene and say that you reject the agreement in principle. This is not at all comparable to a situation where a shareholder would give a mandate and then let the two parties involved negotiate freely and work things out amongst themselves. This is much more intrusive than that.
On May 23rd, 2013. See this statement in context.