Hi, blue. You can add another user by upgrading your version to Plus, Essential, and Advanced. I'll explain this through.
The Simple Start, Self-employed, or Solopreneur versions are designed for one-person businesses. To add additional users, please check our pricing page to upgrade your version based on the number of users you wish to add.

For your reference, here are some articles that can provide you with more detailed information about the various access details for users and how to manage them:
Also, to enhance your QuickBooks experience, or if you need assistance with the task process or managing your account, feel free to collaborate with our QuickBooks Live Expert Assisted team. These specialized professionals can help you with anything you need for your business.
Furthermore, I'll add this helpful page that contains articles you can use if you need specific guidance on performing your task: Browse all articles.
Don't hesitate to reach out if you have any needs or questions about managing your users or tasks. We're always here to help.