Joining the thread to share additional insights. QuickBooks Online(QBO) allows a maximum number of billable and accountant users you can add depending on your plan.
Below are the list of user limits in the program:
Simple Start = 1 billable user and 2 accounting firms
Essentials = 3 billable users and 2 accounting firms
Plus = 5 billable users and 2 accounting firms
Advanced = 25 billable users and 3 accounting firms
If you need to add more users or accountants (maximum of 3), you can upgrade your plan to increase your user limit and invite more people to use QuickBooks. Here's how to upgrade.
If you need further help with user management in QBO, please post here again. I'll be more than glad to help you out. Have a nice day!