There are no individual logins to Web Monitor just 2 roles; Administrator
These are the same as the 2 roles offered by miniDBA Server. An unlimited number of users can log into miniDBA Web Monitor using these roles.
Passwords can be set after installation in the web site's web.config file, in the root of the web site directory. Permission to this directory and file should be restricted so end users cannot change passwords.
The administrator role can access miniDBA Server options and add\remove monitored database servers.
The reader role lacks the permissions to change anything about how Web Monitor or miniDBA Server runs. All they can do is see all the performance data.
Note:- increasing numbers of readers puts negligable extra load on the server so is safe to give reader access to potentially large numbers of users.
Read about Web Monitor Settings here