And stop guests from viewing threads (asking them to log in), will it not affect the SEO?
Or is there no difference between doing that and allowing all guests to view all threads and changing the bots back to guests?
SEO-wise, no, because bots will still be able to crawl and index your pages (supposing you move them to a usergroup with such permissions).
But on the other hand, guests won't be as happy. Restricting content to registered users is not a good idea. You will probably drive more users away than get them to register.
My forum mainly consists of tutorials, learning etc. It is not a "chatting" forum, it is somewhere for users to learn. Would that make any difference?
Another reason not to restrict content to registered users. That's exactly the type of content that should be easily accessible.
But then they end up leeching everything, since there is no point for them to register as they can already view the content.
That's not a good way to think things through. You need to see it from the user point of view, not as an administrator.
Considering you have a tutorials forum, I'm pretty sure most users will find you through search engines such as Google (which you have given permission to crawl and index your website to). And while searching, they might find a thread of yours. But when then try access the thread, they are not able to read it because they need to register. What happens now? They press the back button and keep searching until they find a website that doesn't require you to register to view threads.
Good point, I thought about it from another point of view and decided to take away those permissions, thanks.
Apart from that I don't think it's a good idea at all to have the bots in a user group with posting permissions.
This is called cloaking, if Google find out you're doing it, which they do, they'll just make sure you don't rank in the results at all.
If you allow Bots to crawl pages that aren't accessible for guests, people could just use the Google cache or change the useragent of the browser to the one Google uses to see the page. And, like Matt says, Google doesn't like that very much.