You can create users via "addUser", update existing users via "updateUser" and delete users via "deleteUser".
There are many ways to list users based on their login "getUser" and "getUsers", their email "getUserByEmail",
or which users have permission (view or admin) to access the specified websites "getUsersWithSiteAccess".
Existing Permissions are listed given a login via "getSitesAccessFromUser", or a website ID via "getUsersAccessFromSite",
or you can list all users and websites for a given permission via "getUsersSitesFromAccess". Permissions are set and updated
via the method "setUserAccess".
See also the documentation about Managing Users in Piwik.