You may set the "Default value" for the "Visibility" field in Studio->Forms App->Posts module->Displays->Add Posts area (example for the "Posts" app). And don't please touch the "sys_privacy_groups" table. It may seriously corrupt the UNA's work.
Ok I found it but it is a manual text box.If I write there Members for example do you think that it will work?
May be I need to write the ID from the sys_privacy_groups :)