Here are some login technique samples:
You will need a central data base to record transfers of money from one user to another, one record per transaction.
A spreadsheet for that purpose would typically have these columns, one row per transaction:
- timestamp
- From account
- To account
- money amount transfered (a positive number)
- comment
In such a scheme, a user would only be able to add rows transferring money from his account to other accounts, and would have to wheedle other users to transfer money into his account (by working for them or selling them stuff.)
A separate table could be used to map between secret account numbers and account holder names (ABG, App Inventor Foundation, etc.)