If you'd like to add attendees without them having to go through the sign up process or payment checkout, you can add them manually.
Head over to your meetings page and you'll see action items directly next to each meeting / event. Just click on the plus sign pictured below:
You'll be prompted with a form where you can then manually type in the attendee's name and email address. After being added they'll receive all of the automated emails that we normally send. You can view those emails here.