Ahoj, Zdenek.
Certainly you can... but it won't be easy. Adding new fields to group creation form is relatively easy. But you must also create new records in database table to store them. And of course write right code to send field values to db... and acquire them on group page.
Since we don't support customization, I can't help you with that. But you may consider to hire a developer who could help you with:
www.upwork.com/groups/jomsocial
Ahoj, Zdenek.
No, wed on't provide documentation for customizations.
I think that best choice would be _community_groups.
Ahoj, Zdenek.
But actually where you want display it? In group?
Ahoj, Zdenek.
But on what page? Web site consist with numerous pages... which page do you mean exactly?
Or just on ANY page?
Ahoj, Zdenek.
No, this code works only listing pages.
If you want to display date from db on any page you need to write own code to display it.
Need to fetch data direct from database.
This documentation should help:
docs.joomla.org/Selecting_data_using_JDatabase