That's correct. The system tree can only import and sort machines based on their structure as an object in AD, not based on the user's assigned group.
Maybe some sort of (convoluted) TAGGING based on User Names, with sorting on the TAGS?
That's gonna get complicated very fast, but if your environment is small enough, perhaps that might work for you.
egenlauf, i know you can restrict AD users login to certain machine at certain time. That's the only association of users to systems I can think of in Active Directory. I think you can evaluate your strategy again for importing systems from AD to ePO. I would focus more on organizing computers systems in AD OU. Then setup System Tree in ePO accordingly. I don't think ePO integration with AD doesn't go that far, meaning it's not able to see whether a user is restricted to certain systems. Even if you sync with AD using a domain admin account, the information just won't be there. ePO reads basic user and system properties from active directory.
i would focus more on organizing the systems in AD.