You need a release if ... A properly drafted release basically shields you from lawsuits over two things: (1) you're using someone's image to sell or endorse something; or (2) using the image in a way that harms the person -- it invades the person's privacy or defames the person or otherwise gets them so upset that they call a lawyer and go after the publisher of the photo and sometimes the photographer.
You do not need a release if ... You do not need a release to use a person's name or image for informational purposes. An informational (or "editorial") purpose is anything that informs, educates, or expresses opinions protected as freedom of speech. So if you have a section of your website such as "About Our Members" or you include the images in your non-profit newsletter -- for example, "Members Protest Disney World Mouse Exploitation," then you wouldn't need a release.
Finally ... although it doesn't have the full legal punch of a release, you can always prominently post your photo policy at group gatherings -- a statement such as "We'll be taking photos at our event and posting them at our website. If you don't wish to be included, please inform the photographer."