Custom scheduled report
Is it possible to make a custom scheduled report, example:show user A and user B pluss billing account on same report.
A report that shows a group of users, not all users, and not a single user.
If you wanted to use SQL
If you wanted to use SQL Reporting Services, you can follow an article Nick Wade wrote some time ago about creating custom reports for DA, its not exactly what you're looking for but is a good base to build from.
The problem you will most likely have though is from the Billing Account perspective as Enterprise Vault stores the SID and not the username itself, so when you run an SSRS usage report or just usage.asp, it does a lookup against AD to get the username from there.
If you were to run a straight query, it would not come out as 'DOMAIN\Username' but instead would show as 'S-1-5-21-1004336348-1390067357-682003330-3552'