According to leaked screenshots from the app, Microsoft says that Cortana on the iPhone can be set up so that it can help with organizing meetings during the day, set up reminders on a Windows 10 PC that can be replayed on the iPhone and more. One tester has already posted up screenshots from the Cortana iPhone beta on the Warenotice site.
Since Testfight is designed to only offer up to 2,000 testers for an iOS beta app, it's likely that only a few people have gotten access to try it out. There's no word yet if Microsoft plans to release a more public version of Cortana for the iPhone before it is officially launched.
Source: TechCrunch, Warenotice