I'm having the same issue. All references say only applies to one Workspace. What good is it to pay an annual plan to not apply to your Trello account as a whole, to all Workspaces you've created? Premium Plan doesn't sound like a deal to me.
As far as I understand, Trello charges for member users in the workspace, that is, if you invite people to your workspace and add them as members and not as guests, it will charge you for each of those members.
