Sometimes we as a group will agree to "donate" a few photos to a venue we visit. This often helps us secure access to areas or venues otherwise unwelcome to photographers or the general public -- while providing images to an organization that may not otherwise have the budget to pay for professional photography services.
So it's barter.
Photowalkers who participate in these events are volunteering to share photographs with the organization or non-profit who invited us.   If you RSVP "yes" for one of these events, your voluntary image donation is obligatory. You got exclusive access with a promise of some images, so pony up. :)
Note: you are not "giving up ownership". Rather, you are providing a license to the venue to use your images. This is typically for marketing or historical record. If a venue or organization demand "ownership" of donated images, we will try to educate the organization's leadership about how image rights work.  If that doesn't work, we will not attend that event.