Like the Windows Insider program, the Office Insider program is an ongoing preview program for Office 365 subscribers (both Windows and Mac), providing early access to the latest Office features. It's open to anyone who has an Office 365 subscription (administrators need to enable First Release on business accounts before business users can install the Insider Builds).
If you want to file bug reports, you'll need to "upgrade" to an Insider build. Then you can go to File, Support, Contact Support. I highly recommend doing this if you feel a behavior is a bug.
Monthly Channel (Targeted) is the original Insider program. Users in this ring will receive an update about a week before the official production release each month, and then the official production release when it goes out (2 updates per month). These two are generally very close in terms of features and fixes.
Office Insider is a new level that delivers weekly updates from the development branch. There will be features available to users in the Insider ring that won't be available to Monthly Channel (Targeted) or Production. Because this build is from the development branch, these users always be a version ahead of the official production release.
You can check to see which update channel you are currently using in File, Office Account (or Account in Word or Excel). The update channel is listed below the suite name and the version number.
Join Office 2016 for Windows Insider
If you'd like to join the Office Insider program and have a consumer subscription (Home, Personal, or University subscription) go to the Additional Install Options page of My Account and choose an Insider option from the Version drop-down menu.
Office 365 business and enterprise users get the Insider Build through First Release program. To switch from Insider Slow to Insider Fast, you'll need to set the following registry value:
Microsoft is changing the channel names (again!). Below are the valid values for UpdateBranch. At this time, either the old and new updatenames will work.
(was Current Channel)
|Insider Slow or Monthly Channel (Targeted)
(was First Release for Current Channel)
|InsiderFast||InsiderFast||Office Insider or Insider Channel
(was Insider Fast)
(was Deferred Channel)
|Targeted||FirstReleaseDeferred||Semi-Annual Channel (Targeted)
(was First Release for Deferred)
Join Office 2016 for Mac Insider
To join the Office for Mac insider program, open an Office 2016 for Mac application then click Help, Check for Updates. In the Microsoft AutoUpdate dialog box, select Join the Office Insider program to get early access to new releases check box and choose between Fast or Slow updates.
The iOS, Android and Windows Mobile apps (Word, Excel, PowerPoint) are also available to "Insiders".
More information on the Insider program is available at Be an Office Insider
What's new for Office Insiders (Microsoft Office)
Update history for Office Insider for Windows desktop
Overview of update channels for Office 365 ProPlus (TechNet)
More information on the Insider program for Office for Mac is available at Welcome to the Office Insider for Mac community!