You want to set up an event where one of the attendees (e.g. a presenter) is not a member of your organization.
Use Add a Person option to create and invite an external user:
On the Edit Event page or the People Dashboard page, click on the Add a Person button in the top-right corner:
Enter the person's name and email into the dialog:
Select External for the Role option
Click on Add and Send Invite button
Now you can go back to your Edit Event page and reference the new person in the appropriate field (e.g. as a Presenter).
External users cannot login to the site, but they still get calendar and email notifications if Settings ⇒ SSO and Security ⇒ Allow external users to receive notifications and calendar invites setting is turned on.
Note that only your site admins have the privileges to change the system settings. Only organizers and admins can add new users, including the external ones.