The Management Client is in itself the natural place to add and manage users. (No API or development required.)
The Add User plugin is a Smart Client plugin, it makes possible to create users from Smart Client and avoid using the Management Client.
http://doc.developer.milestonesys.com/html/index.html?base=samples/scadduser.html&tree=tree_1.html
The Add User plugin sample uses Configuration API which is supported in c-code XProtect servers. The Configuration API methods can be used in standalone as well as in any kind of MIP plugin.