Upgrading to Pro is on a per seat and per Pro Workspace basis. When upgrading to Pro, you will need to upgrade all the Creators in your Workspace as well. Additionally, upgrading one Workspace account to Pro does not automatically upgrade all your accounts in other Workspaces you may be a part of.
All users in a Pro Workspace members need to be in a paid seat, either as a Creator Pro or Sidekick. Read more about the difference between different roles here.
Workspace members can be upgraded via the Workspace Admin's Workspace Settings.