How to get field value of form before submit?
If I'm not wrong, you want to update the other fields based on group_name entered by user. To achieve this you can use AJAX API provided by Drupal 8 Core.

If you don't know how to use/work with AJAX API, you can have a look at Examples for Developers module which provides working examples for many of the functionality. To see AJAX API Example, you can install "Form API Example" Sub-Module from Examples Module. Feel free to go through Examples Module code for any help you need.