I have a Business Class subscription, and I want to add members to my boards without incurring in additional charges for new subscriptions.
You will be charged for any members that are on more than one of your Business Class team's boards.
So long as you don't add the members to the team, and you don't add any member to more than one of the team's boards, you won't be charged.
See How Billing Works with Trello Business Class
It looks like you're new here. Sign in or register to get started.