Greetings Tracy ~
Designing processes always means doing a little adaptation between the processes and the tools. So out of the box, this works just fine with the following scenario:
~ One single instance of DotNetNuke.
~ Each dealership would be it's very own site (ie. www.dealership1.com, www.dealership2.com, etc), with it's own Administrator(s)
~ Each dealership would have it's very own user base, roles, file storage, etc (ie. they truly are logically separate web sites)
~ The "SuperUser(s)" (also called "Host" users) would be able act as an Administrator of any site, ie. they can see all the sites and have global permissions to change settings which affect all sites, add modules, etc.
There are ramifications of these choices that require you to think through what you are trying to accomplish and how much trouble it would be worth to you to change either the tool or the process. It is unlikely that one application (DNN or other) will accommodate all the abstract process choices, but we usu sally find that with a little analysis DNN will work well for almost any scenario (once it is well understood).
If I understood your first post, you were looking specifically to give User Management (only) to certain users? To clarify, under the scenario above it would be possible (in DNN 5) to give access to user management ONLY within each site. If you wanted one person to do this (who you did not want to be a SuperUser), they would require a separate user login for each dealership.
Does this make sense?
Cheers
EDIT: The key here is to first define at which level you desire a "site" to exist. If it is at the dealership level, the above scenario works fine. However, after re-reading... it sounds like you may really be wanting the "site" at the brand level (ie. Chevrolet). In this case, then Sebastian's comments are correct and what you are really asking for is some sort of "dealership management" application to help categorize the folks signed up at each dealership. Out of the box DotNetNuke provides for the capability to administer ALL users in a site, rather than just a subset. This is why the question of level is important... is a site a dealership, a brand (Chevrolet), or Corporate.