One of Java's strongest benefits (if not the strongest) is its Write Once, Run Anywhere (WORA) philosophy. An unfortunate side affect of this is that there are certain things that vary too greatly from platform to platform to handle in a unified way. User and user group definitions likely fall within that category. To the best of my knowledge, Java does not support the concept of user groups. Instead, it uses Security Policies to apply restrictions and grant permissions. I do not have a lot of experience with such since most of my work is with Web Applications and the web container provider authentication and authorization services for my applications. I suggest you look at the Sun Security Features in Java SE tutorial. It may hep you accomplish what you want to ultimately do.
Others may have other comments and suggestions.
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