You can send offers to eligible watchers from your Active items page, but only once per eligible watcher (meaning they haven't disabled the option to receive offers from sellers). Generally that's only when eBay is prompting you that it's an option. There is no way to send an offer to someone who simply viewed your item.
When you send an offer on an item, there is a click box at the bottom of the offer to make it automatic anytime there are eligible watchers. From then till sold, it will automatically send offers to watchers on that item.
