Since this is an ad hoc group, and all this individual does is add havoc, there are few steps to take. You have already taken the most important one, namely to talk with your manager. He could have used his influence and power to help bring this individual into line. Since he opted not do so, your disruptive associate may feel even further emboldened.
The next step is for the four of you to meet with this problematic associate to explore possible ways to work together. One approach is to ask how he would like to work with you. He may be a creative loner who could be more effective by emailing his inputs to the group. If you let him come up with a way to constructively work with you, he just may be able to do so.
If he continues to be a disruptive force, then you and your associates need to revisit your manager, give him an update, and let him see how the project is suffering. Although your group is studying benefits, there will be no benefit unless he takes some corrective action.
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