PSA as I've only just found this out while adding another share onto my plan:

Adding a Share onto your mobile plan now cost $29.99 each and is called the "Sharer" plan, the old $25.99 share has been grandfathered. 

The difference being is the share can now get their own Spotify Premium account instead of it just being 1 spotify premium plan per group.

There is no detail of this on Spark's website (yet..I hope), but given its $10 more expensive than Vodafone it kinda seems like a way for them to cover the cost of adding Spotfy so its all good if you use it.