It can be positive, neutral or it can drag the site down. I would say this depends on how the guestbook is structured and on the level of moderation you apply. For example:
- Does the guestbook display one comment after another on the same page? - If you have many comments, is the pagination used to paginate through comments? - Can each comment be also accessed on its own page? If so, do you have many one/two line comments that would make such page "thin"? - Do you moderate comments to fix the language used (e.g. spelling)? - Are all your comments in a single language or are languages mixed? - Do comments allow external links? If so, are these links nofollowed?
All these factors may influence whether a Guestbook will be a positive (or neutral) influence to the site or whether it will drag the site down.
Some of my clients demanded guestbooks for their sites. I of course complied, providing all caveat that they must moderate the guestbook constantly. Those who did so found benefit. Those who did not soon found their rank and presence in the serps disappear.
To specifically answer the OPs question: If there's a url that can be crawled, it will be crawled, and that content, guestbook or not, will be indexed.
The most successful guestbooks (by my clients, that's all I can speak to) have explicit nofollow noindex on their guestbook AND have done their moderation for content and spam, etc.