I have presented in the past something I called Rapid Release Planning. While I stand by this method for teams that have a history of delivery, I have found a much better way for new teams or teams with no previous history of delivery. It’s called Silent Grouping.
If you are interested in possible mechanics, please follow the link above. The only guidance I would provide is that I do not start out by using any numbers. This seems to work much better in that all I ask is for the team to place the stories from smallest to largest. Natural groupings occur when you do this and then you can then use these groupings to draw lines on the board that would correspond to points from the natural groupings.
The real importance is that by utilizing this technique a newly formed team is able to get stories pointed much faster than anything else I have ever found. It keeps people focused on relative size and avoids in depth discussions about acceptance criteria.
More importantly it can be used for other single dimensional relative measures, story value, for example. If you then take these two relative values you can come up with a quick, easy and painless way to see a relative ROI for each story.