You can add a guestbook to your web page by clicking the 'Tools' link on the toolbar in the WebStarts page editor view and then choose 'Guestbook' from the drop down menu. This will drop a Guestbook on to your web page as indicated by the 'Please Sign Guestbook' text that appears. You can then drag it and size it accordingly and can edit the text size, color and style using the font attributes located on the toolbar.

Please note that the Guestbook will expand as new comments are added on it. Therefore it will make sense for your Guestbook to be at the bottom of your page.