If your campaign is configured to NOT have participants login/authenticate:
Manually via participant: Your participants can enter the same email into the signup form and their existing information will be returned.
Automatically via grsf_email URL parameter: You can provide your participants a URL with a grsf_email parameter. If they clicked on the link and landed on the webpage, the person will be automatically logged in (or added as a new participant, if they did not already exist).
Automatically via embeddable elements: To automatically authenticate the participant, use the data-grsf-email attribute with the GrowSurf embedded form