I tinkered some more with this, I found that if I scheduled Drop-ins then I could invite Free Agents. Drop-in events seem to work well as it doesn’t involve identifying opponents. Only issue I have is that the event titles I used don’t appear any more, even though I included that in the event description. (Event title optional for Drop-ins)
I used event titles “Game XX / YY Games to go” so I’d like for the event title to show that in the invites, dashboard, calendar, schedule and event detail screens, as it identifies each event uniquely.