Connect with and learn from others in the QuickBooks Community.
To ensure that you're able to add a user, it's best to sign in to your account through a web browser and then go in the settings.
QuickBooks Online allows you to invite different users through the Manage Users window in a browser.
To add or change user types in QuickBooks Online through a browser, you can refer to this detailed article: Add, delete, or change user access.
That will do it. I'm only a comment below or a post away if you have any more questions.