Of course, @Dawnmarie1 . Just write your message in the inbox you used when communicating with the guest before and during the stay. It still works after checkout. Contact information once again gets masked, but you are able to write to the guest, and they are able to write to you.
If it is something a bit negative (a note on how to be a better guest in future?), you might want to wait until after they have written their review of you. If it is positive ("Thanks for the great decorating tips!") then by all means do it now. 🙂
I have never tried to edit a review (during the edit period) to add a private note, but, with the messaging still active, it really isn't necessary.