Enroll in Office Insider when using Office 365 enterprise

Method 1 : using the Office deployment toolkit
Download the office deployment toolkit
https://www.microsoft.com/en-us/download/details.aspx?id=49117

Extract the file to a directory.

Edit the configuration.xml

<Configuration>
  <Add OfficeClientEdition="64" Channel="InsiderFast">
    <Product ID="O365ProPlusRetail">
          <Language ID="en-us" />
     </Product>
   </Add>  
 </Configuration>

Now install Office 365 with this command line :
“Setup.exe /configure configuration.xml”

Method 2 : Edit the registry

Start the Registry Editor and browse to the following key Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\ 

• Create a new key with the name Office 
• Under this key Office you create another key named 16.0
• inside 16.0 create another key with the name Common
• inside Common create a KEY with the name OfficeUpdate
• inside the OfficeUpdate key create a String Value named UpdateBranch
• Edit UpdateBranch and enter “InsiderFast” as Value Data

Go into an office app and start the update process, Update Options -> Update Now

After update it show Beta Channel (= the “New” Insider Fast).