Haystack searches your phone's contacts to find which of your friends are on Haystack. Once it finds the relevant contacts, Haystack uploads their information to Haystack's servers.
Add your friends to your Haystack to stay up to date with their latest contact details. When your friends change jobs - Haystack will update your phone with their latest contact details automatically.
You can find your friends on Haystack at any time by going to the "Settings" and tap "Search phone's contacts".