@marcime:
Thank you for your quick response.
I not pretty sure wheter you got me right in the first place. It is not about an additional Group Setting but about a general restriction. For my page I don’t want my members to set up private or hidden groups at all.
Every group should be public. 
@ all:
Maybe I can amend the source code. Does anybody know how?
Thank you in advance and kind regards,
Pasen
If you have a child theme of bp-default theme, the simple way would be to copy over:
a) http://buddypress.trac.wordpress.org/browser/tags/1.5.4/bp-themes/bp-default/groups/create.php
to wp-content/themes/YourChildThemeFolder/groups/create.php
- above line 90 per linked file add “
- below line 106 per linked file add “
b) http://buddypress.trac.wordpress.org/browser/tags/1.5.4/bp-themes/bp-default/groups/single/admin.php
to wp-content/themes/YourChildThemeFolder/groups/single/admin.php
- above line 69 per linked file add “
- below line 87 per linked file add “
Note that if you as Site/Super Admin don’t even want to create a hidden or private group yourself, then delete lines 90-106 from create.php and delete lines 69-87 from admin.php