Time is a scarce resource. Your colleagues will appreciate your ability to quickly focus everyone's attention on the reasons for the negotiation and the benefits of a successful resolution for all. You could say, making a final decision about the direction of this project is critical to meeting our deadline, getting this product out the door and hitting our revenue targets. For that reason, I have asked you all to convene and determine where we ought to go from here. This said, your colleagues should not resist investing time and energy in it.
Most people dislike negotiating, especially over tough issues.
But those same people may enjoy problem-solving. Present the activity as joint problem-solving so you can effectively engage them in a conversation around the topics you want to negotiate. Then, you can work with them to better understand and attack the issue. Start with the other side's main points or concerns. To make the meeting really feel like a joint problem-solving session, begin by raising the other party's critical needs or concerns.