Thanks for raising this question around adding the external contributor issue you're running into. Unfortunately, you won't be able to add an external contributor to either, as they aren't intended to be added to spaces or permission groups. External contributor will only have access to private or secret externally accessible social groups wherever they've been invited to and will be unable to interact within other places.
I'm not certain which specific Jive version you're seeing this on, but for your reference I've linked to the Jive 8 documentation that summarizes this:
Let me know if you had any other questions on this.