Maurício
Thanks, that explains that CustomerIDs does work how I thought it did. It's no good - we have far too many customers to do this with. With only a dozen or so I can understand this being an effective solution, but it's unmanageable and unwieldy for the amount we need to be dealing with.
Hi, we are using, at customer creation page, the attributes CustomerID and CustomerIDs. CustomerID we are filling with the same value of Username and CustomerIDs is semi-colon list of CustomerID of other customers of the same company. A collegue at the job did this “trick”. I think it also had to enable this CustomerIDs field, changing ~/Kernel/Config.pm and ~/Kernel/User.pm