Twitter autofollow feature allowed users who follow you to follow back right away. autofollow was a feature that was not a public feature. It was granted to limited set of accounts. autofollow feature allowed spammers following other accounts. So, Twitter discontinued autofollow because this behavior.
Still, if you want use auto-follow twitter feature to follow people on Twitter, consider these tools.
twollo
twollo allows you to find and follow twitterers with similar interests to you automatically, for free of cost. Twollo allows you to Target Twitters and grow your network.
twollow
twollow will automatically follow people based on keywords. it has a 7 day free test drive account, after that you can upgrade to paid account.
Another good option for auto-follow fans, there are several services available like socialoomph, tweetingmachine, and socialtoo