When you schedule events within an event series, you can choose to hide the events from your schedule. This will make the series private and only visible to the people that you share the event with.
First, create your event series and schedule at least one event in the series. Next, change the "Events to display on the Schedule page" option to "Hide All Events".
This will hide the event from your Schedule page. You can get a sharable link to this series by clicking the "Share or Embed" button.
You can then share this link with anyone that you would like to register for your series. If you have another website, you can also embed this event series on that website.