Each site that is part of a SiteStack has its own set of users and therefore its own set of administrators. So if you use the standard ‘admin’ account then each site will have its own ‘admin’ account.
Themes and Plugins
There are two ways that you can set up the handling of themes and Plugins. You can have multiple domain share themes and plugins or have a unique set of plugins and themes or any domain.
Shared Themes and Plugins
With the shared option each site that shares Themes and Plugins has the same set of themes and plugins installed. Each site does NOT have to use the same theme or plugins. They just have the same set available. This is similar to the way WordPress.com works. This option is best used if all the sites belong to (or are managed by) the same group of people.
Unique Themes and Plugins
The unique option works pretty much the same as a single install of WordPress. This option works well if someone just has one site.
Remember you can use multiple shared directories so you can set up groups of sites that are managed by separate people.