You click on the options button, and then You see a look like this.
What will be shown - and if the options button will be shown, depends on the settings You have done.
Remember that the appearance of many things, depends on the settings.
User groups can be selected by :
Either check "All groups"
Check out the wanted User groups ( multichoice)
When selection is done, You click "done", and You are back to the calendar.
You now, only see the events that have an eventowner/user that belongs to one of the selected usergroups.