Doing so allows them to use the receipt capture feature. You can only add limited users depending on your plan. For more details, please see this article: Learn About Usage Limits in QuickBooks Online.
Once done, please have your employees accept the invitation. Then, inform them to install the QuickBooks Accounting: Invoicing and Expenses app on their phone to scan a receipt.
If you have other concerns or questions about setting up your users, you can always share them with me. I’ll be around to answer them for you. Keep safe!