The page on active.com says that registration closes at 8 am Friday. But that may only be for pre-registration. The Bike Roswell page says "Registration is $35 ($40 if received after July 3, including day of event)" which seems to indicate that on-site registrations will be accepted. But if you want to know for sure, send e-mail to Kristin Stanley who is the event organizer (the info packet was sent by her and contains her e-mail.....I won't post it here).
I drove the first part of the course today. It winds through Huntcliff (just south of the Chattahoochee) and is very hilly. The first leg (15 miles) could really be tough for some riders, like me!