FigJam is completely free until 2022. But for the future:
I don’t see any mention of “collaborator” in FigJam pricing Frequently Asked Questions on Pricing — but I would suspect this means anyone: both editors and viewers.
As far as I understand from this article Open sessions: jam with anyone, anywhere — yes, as long as you are on the Pro plan you can create a Session and invite everyone there for free.
Both of these would probably be clarified in pricing when FigJam becomes paid.