The Role system in ez publish allows you to set up such access.
Edit a Role (or create a new one), and set the following permission: content; edit; Class(User, User group), Subtree(<part of user section to manage>)
When editing the role, click the small "New" button. Select "content" as your module and click "Allow Limited". Select "edit" as the function and click on "Allow limited".
In the class box, select User group & User. Section can be set to "Users". Owner to Any. Then click on the magnifying glass under Subtree and navigate to the part of the user section that you want to manage with this role. Save all changes and don't forget to assign this role to a user and/or user group.
You can do the same thing for the "create" function of the content module, if you want the maintainer to be able to create new users and user groups.
