In association with the site is a ning, with is accesible by invitation only. One needs to go to the contact page at The Plantation Letters site and find the email addresses for the creators in order to ask for an invite. I've sent in a request and will let you know if there are any problems.
ALSO, they say......."We need VOLUNTEERS! We have more letters about slaves, women, and children on antebellum plantations than we can transcribe. If any teachers or other readers would be interested in helping to transcribe and get more letters online, please contact us. Transcribing assistance would make a great service learning or class project for students with contributors gratefully recognized on this page."
Image: Nick Sherman/Flickr Katie