How to track errors when distributing Android Application to Alpha Testers?

I recently released an application for some users to use my newly made Android Application. However, they are having trouble when they perform the Facebook Login feature that I have made.

It seems to work fine on my phone, but on other phones it seems to crash. I would like to know what is causing the application to crash so I can further debug this. Is there anywhere or anything that I can use to debug this problem further?

I have looked at both the Facebook Developer Console and Google Play Developer Console and neither seem to show or point me to where my error is. If anyone could help that would be great. Thanks!