We have a group of users at an external location that have been added as guest users to 365 with B2B and they have been added to BC. The authentication email shows with the #EXT# format and users are not able to login to the Android app. I created a deep link and launched it through adb on several versions of the BC app down to 2.8 (on both an emulated device and a physical), but was not able to replicate the behavior described in the documentation.
Starting: Intent { dat=ms-businesscentral://businesscentral.dynamics.com/tenantguid }