What would you do if someone turned up uninvited at your party? Look at them skeptically? Throw them out? Now what would you do if someone arrived at your community from a search engine?
Community building is entirely about building relationships that last. Examine your social group, I bet there is an intricate network of ties that connect your friends with each other. New members come in through one of these ties. Like a community, people invite their friends, who invite their friends. Everyone is there because everyone wants them to be.
Search engine traffic is different. I would treat them differently, even skeptically. First, assume any visitors from search engines are simply bonus to the work you’re doing. It should never be part of a community building strategy. Second, I’d push them to build relationships with members quickly, or leave.
Why not put a ‘Just Arrived from Google’ thread on the forum, or a page just for these people? Include a brief primer about the community, hot topics and actions they can take to build relationships quickly.