How does an employee access the Time Sheets to enter their hours once they have accepted the invitation as a user?

Hi jtower, 

You can invite your employees as Time tracking users only. From there, they can use the credentials they created to log in to QuickBooks.

Let me show you how: 

1. From the Gear icon, choose Manage Users. 
2. At the upper-right corner, click the Add user button. 
3. Choose Time tracking only, then, click Next. 
4. Select the employee's name and click Next afterwards. 
5. Enter their contact information. 
6. Once done, click Save.  

In addition, they have two option on how to enter their hours. It can be for the Week or for a Single day.

You'll also want to refer to this article for more information about Time tracking. 

I'll be around if you need more help. Have a good one!