Give Google bot access, but not guests
 

I want to set my forum so guests have to register to do anything, but I still want to let search engine spiders come in. Is that possible?

I would not suggest to do so, your site might get removed from the SE listings. See: Cloaking

AFAIK the only way to go about it is to give guests limited access, but the google bot complete access. This would boil down to guest being able to see a limited number of pages. Google has specific rules for forums and member based sites.

thats my question, how do I give the google bot complete access?

search for the mod "Treat spiders as members"

Be warned like Marco said, it could be considered cloaking.

^ on this forum? :confused:

Do a search in the Mods forums. You may select "all mods" in the drop-down in the search box in the upper right corner. But, as stated, read google's policy so you aren't surprised if your site is suddenly no longer in their search results.