Hi ..... Could you not exchange the word company to And use the word account. Therefore Company A has an account called A********** Company B hascan account called B ********* Then could you then not go to record level and make your setting for each account.
I'm not sure I can call or make a relationship with the accounts (Security). If so than that may work.
This can be done in FileMaker. See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a detailed description of how to set this up.
(You can flag your records with account and/or company names and then use that information in a calculation expression set up in Manage | Security to control which records are accessible and in what way for each user that logs in.)