Principles and differences between various web-access modes are described in section Web-interface access modes.
To configure web-access for a user you should first add web-access for user group, in which the user is located.
Settings from user group's web-access will be considered as default settings.
Then you should add web-access for particular user.
You could either leave default configuration (in this case configuration will be taken from user group configuration), or define some special configuration.
For each user you should define unique login/password to access web-interface.
After this you'll be able to enter web-interface using specified login and password by address:
https://your.server.name/