There could be a limit of the total number of Insiders for a specific app preview, so you will be informed if and when you can join the preview. (windowsphonearea.com)
At the time of writing this story, the Windows App previews program is available for a small group of Insiders and core applications such as Windows Camera, Microsoft Photos. (windowslatest.com)
On Monday, Jared Spataro, general manager for the Office marketing team, took to the Office Blogs to announce that the Office Universal apps preview for Windows 10 would hit phones by the end of the month. (bgr.com)