Unfortunately it isn't 'easy' to add another family member.
You'll have to cancel your current family plan. Once your account drops to the free plan (you can ask Spotify's support service to do that for you, if you can't wait) you can subscribe to the family plan again with the extra user.
Then you'll have to invite all the users (and the new user) again.
Your account will drop to free on the next billing date. If you go to your account it will say something like: "Your Spotify subscription ends on year-month-day and will revert to Free."