Ah, I misunderstood your question; sorry.
The good news are that Guests come along on the ride for free so you never have to pay for them. They can be users/collaborators, generally, but not creators, generally. For example, if you invite them to a project, they can update the values of custom fields you’ve created, but they cannot create custom fields, nor edit the definitions of those custom fields.
Full info here:
Thanks,
Larry