Hi Davidv
I help work on the app and can give you some insight to this.
The App uses the Email Address to find the right files and creates a link to them through the app. It doesn't lookup anything else and, at the moment, can ONLY send documents through to the In Tray of the file it logs into. So it doesn't use the "second login" of ARL.
There are considerations to change this to make it more accessible to link to an In Tray without a login, rather like the Unique In Tray Email Address that you can create and give to people to send docs to your In Tray without actually giving them a login. We're still talking through the logistics of this though and have no concrete plans to know if/when this'll be delivered.
In the situation where there are offsite/remote workers that the business would like to use the app, but given no actual access to AccountRight; currently the recommendation is to create a login for them that's connected to the file but no actual usable access in AccountRight. (You can use my.MYOB to send an invite without creating a User ID in ARL so if they try and use the Email & Password to log in, they won't get through the second login but can still use the app to send docs through).
I hope this helps answer what it can currently do. Let me know if you have any extra questions on this!
(Note: I use AccountRight as an example as you posted in the AccountRight Ideas Exchange)