How to administer the server (pod) of Diaspora? Are there user management, for example?

You can add some users to the admin list in config/application.yml, variable admins (line 73 in my file/the standard file). This will give the added user some additional option in the dropdown menu under your login, called ‘admin’. You can search for users, get an overview of failures on the server etc., but you don’t seem to be able to manage the users.