I've got the information you need about setting up multiuser mode in your client's Mac version, @greg. Aside from PC, this is also a supported feature in Mac 2019.

First, your client must buy a multiuser license of QuickBooks. Then, they can add users to it. I'll guide you how. 

1. Go to QuickBooks from the top menu.
2. Select Manage License.
3. Choose Buy Additional Licenses.
4. Follow the on-screen instructions to buy the additional licenses.
5. Go back to QuickBooks from the top menu.
6. Click Manage Licenses.
7. Choose Sync License.
8. Install the program on each computer your client want to use to work on their company file.

Second, please have your client install QuickBooks Server. This way, other users on their network will be able to open the shared company file. For more tips, see the Where are you going to store your company file? section through this article: Multiuser Checklist. 

Third, have your client create user accounts. This includes the usernames and passwords of their users so they can access the company file. Just go back to QuickBooks from the top menu. Then, select Preferences or Company and choose Users and Passwords. 

Fourth, please have your client set up the multiuser mode. Here's how: 

1. Open the Multiuser folder.
2. Drag the QuickBooks Server icon to the Applications folder.
3. Choose Authenticate to modify the Applications folder if prompted. 
4. Enter the Admin username and password.
5. Hit OK.
6. Launch QuickBooks and open your company file.
7. Go to File from the top menu. 
8. Click Enable Multi-user Mode.

For detailed instructions, see Step 5 through this article: Install and Set Up QuickBooks Mac Desktop. 

Once done, QuickBooks Server will start automatically. Also, your client's users can access the company file simultaneously. It will be listed under Shared Files on the No Company Open window. 

Additionally, I'd suggest visiting this website: Mac 2019 User Guide. Then, please go to pages 26-30. From there, you and your client will learn more about the QuickBooks Server and the other tasks to perform when using the multiuser mode. 

Please get back to me if you need further assistance. I'll gladly help, @greg.