Is the $39.99 cost per user or per account?

Accounts can shared forms/access to forms with other accounts as sub-users:

https://www.jotform.com/help/232-How-to-Share-Forms-with-a-Sub-Account-User

Each subscription level can add a certain amount of users:

Bronze: 3
Silver: 10
Gold: 100
Platinum: 1,000

Forms created by sub-users in the shared folder would have the same limits as the account that shared the folder.  This way, you can have a single upgraded account and allow for other Free accounts to have access.