MANNNNN!!!!!!! I did not know there was one to sign or I would have!!! I'll just have to do it next time I am there in a year or two!
When you find the guest book check out the signatures and dates of others, I wonder how far back they go now. I'm sure more than one book has been filled. You should find it on the podium near the alter.
The first time I went to Portsmouth was just after the last residents had moved off the island, approx 1969. The tables in the houses were set for dinner, clothes hanging on lines, kinda like they just picked up and left.