The primary plan holder can decide how much of the provisioned 75 GB he/she wants to share with each of the add-on members. The 5GB which is mentioned in the release refers to the additional data which is provided to the add-on connections over and above what is shared with them by the primary plan holder.
If they have activated the plan less than 6 months back then they get price protection for 6 months since the date of plan activation. If it was activated more than 6 months back then they will be shifted to the nearest value plan from the next billing cycle onwards.