It wasn't possible in UNA 12.1 as I remember, in later version we've added such functionality, however it's not directly possible from "Add new" form, but you can specify specific Group in "Visible to" field during Event creation.
Thank you Alex.
The visibility option setting also works in 12.1 but the event is not listed with the Events link at group's page.
I tested at online.me and yes there it is listed. May be upgrade time for me is coming :)
Ekran Görüntüsü - 2024-03-29 09-01-43.png