I sent an email to customer service this morning.
"What type of pen does Shutterfly recommend using for the photo guestbook? I'm worried about it smudging or smearing."
Their response:
"Thank you for contacting Shutterfly.
You can write in the book with a regular ball point pen. A paper mate pen with a medium tip also works fine. Shutterfly uses an 80-pound paper stock for the photo book and it is an archival with an acid free glossy finish. Please note that it is not a high glossy finish. We request you to use a similar pen for the better result.
We thank you for choosing Shutterfly. Please let us know if we can offer any additional information."
that answers that :-)