I've been asked to participate in such an event and now want to understand its origin and purpose. pdf 
A Collective Story Harvest enables us to deeply connect with and learn from the experience in our community, team or organization. A number of stories are shared in small groups and we work with a set of specific themes to harvest each story. Each of the participants either harvests one of the themes or is a witness during the storytelling and then shares back to the storyteller and small group. Finally, we come together to converge our learnings across all the stories.
We’ve found that group harvesting takes time – at least 90 minutes is the minimum time needed. Keeping the storytelling to around 30 minutes is advisable otherwise it is easy for listeners to become overloaded.
It is best to have those directly connected to the story on hand to tell it, and it can be more interesting to hear from more than one person involved in the story. More voices add depth and richness, as well as a variety of points of view.
To prepare as a storyteller, set some time aside to do a little bit of writing. Think of your story topic and make some notes along these lines: hereʼs who I am, hereʼs who is involved, here is the challenge that faced me, hereʼs what happened, where and when, hereʼs where we are now.
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The organizer says, "Collective Story Harvest is known within parts of the change making / change management community but not well known within the IT community yet. It has been used a lot within the European Union and people in the US and Canada use it in business and social leadership issues. I like it as a way to use stories to support collective learning both for the listener and the story-teller."
Lessons from the Manifesto will be derived from my and other's stories at the OOP conference.
Alpbach Summer School on facilitation and participatory leadership explored this and related activities.