Microsoft first announced the Office Insider Program for iOS last August, with the first build coming in January. Previously, it included Word, Excel, PowerPoint, and OneNote, but as of today, Outlook joins the family.
As with all of the other apps in the Program on iOS, you'll need to apply for an invitation to be an Insider, but there is some good news. Between the original four apps, you were forced to choose that one that you use most, and that would be the one that you could test. Outlook uses a separate form for its registrations, so you should be able to test it side-by-side with the one you chose before.
The firm says that it will be offering spots on a "first come first serve basis"; however, it doesn't note just how many spots are available. You can apply for Outlook on Office Insider for iOS here. If you're accepted, you'll be emailed an invitation to download it from Apple's TestFlight application.