Can I add additional Sub Account Users without upgrading my plan?

Unfortunately, the only way to increase the number of sub users and other account limits is by upgrading to a higher subscription plan. For "Bronze" plan, you are allowed to have a total of 3 sub-user accounts. If you wish to increase that, you'll need to upgrade to Silver or higher.

To learn more about the use of sub users, please do check this guide: https://www.jotform.com/help/232-How-to-share-forms-with-a-sub-account-user 

I hope that helps.