For self-service premium plans (Plus and Pro), workspaces are upgraded, not accounts. There are no premium users in Airtable, only premium workspaces.
When a workspace is upgraded to premium plan, all bases in the workspace will get the corresponding premium features (be it the increased record and attachment limits on Plus, or numerous additional features on Pro). 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 on self-service paid plans (Plus or Pro) are free as well as any person submitting a form on any plan.
All initial and ongoing charges for the workspace are charged to the payment card uploaded by a workspace "owner." Other people accessing the Pro workspace do not pay for their access, and they'll have free features on other workspaces they belong to that aren't on a premium plan.
Airtable plan changes are automatically prorated, accounting for the addition and removal collaborators, as well as plans switches. On both monthly and annual plans, we bill or credit for any changes once per month.
It's typically best to have all bases for which you'd like premium features into a single upgraded workspace. If you'd like certain people to have access to only certain bases, you can add them as base collaborators rather than workspace collaborators.
You can check a workspace's plan next to its name:
Please note that for Airtable Enterprise plans, billing is calculated differently. Please contact your sales representative if you have further questions.