If add user function is replaced by upgrade function in Manage User in all Simple Start clients what must I do to to invite users?

Good to see you here, @ronz-worldonline.

Let me provide a concise explanation of why the Add user function was replaced with the Upgrade button for all Simple Start clients.

The reason it was replaced is that QuickBooks Online (QBO) Simple Start subscription is designed for a single user, along with access for an accountant.

If you want to add a user, you need to upgrade to a higher subscription by tapping the Upgrade button. Feel free to click this link to see what plan fits your business: QuickBooks Online Plans and Pricing.

If you opt to upgrade, you can modify the roles of your users. Check out this article: User roles and access rights in QuickBooks Online.

Don't hesitate to leave a reply if you have additional questions about managing your users. We'll be willing to help. Take care, and have a good one.