Upgrading to Pro is on a per seat and per Pro Workspace basis. When upgrading to Pro, you will need to upgrade all the members in your Workspace as well. You can decide who should have Admin permissions and who has Creator permissions at checkout (or after checkout). Additionally, upgrading one Workspace account to Pro does not automatically any other Workspaces that you may be a member of.
All users in a Pro Workspace members need to be in a paid seat. Read more about the difference between different roles here.
Workspace members can be upgraded via the Workspace Admin's Workspace Settings.