This is how Airtable support explained it to me, which helped clarify things:
In Airtable, workspaces are upgraded, not accounts. There are no premium users in Airtable, only premium workspaces. When a workspace is upgraded to Pro, all bases in the workspace will get Pro features. The cost of this upgrade and any continued price adjustments over time depend on the number of people who can edit (or comment on) at least one base in the workspace. Read-only collaborators or people submitting forms are free.
All initial and ongoing charges for the workspace are charged to the payment card uploaded by a workspace “owner.” Collaborators accessing that Pro workspace do not pay for their access. If those same collaborators have their own workspaces that aren’t on a premium plan, those workspaces will continue to have free features. Workspace plan changes are automatically prorated, accounting for the addition and removal collaborators, as well as plan switches. On both monthly and annual plans, Airtable bills or credits for any changes once per month.