If you have an Organizational subscription account and are an administrator on that account, then you can go into the organizational settings, in Security, and set it to not share data outside your organization.
If it is a personal account, all your maps have to be public. Prior to the subscription based accounts, you could make your maps and groups private. I switched one over like that recently, to a subscription based account and it was quite an experience.
Currently ESRI is providing a base level subscription to those people participating in the ESRI Business Partner program, through the end of 2013, I think.
Hope that helps some.