How to Join a Mac to a Windows Domain
Choose Settings→Users & Groups.
This brings up the Users & Groups page.
Select the user account you want to join to the domain and then click Login Options.
The Login Options page appears.
If the lock icon at the bottom left of the page is locked, click it and enter your password when prompted. Click the Join button.
By default, the user login options are locked to prevent unauthorized changes. This step unlocks the settings so that you can join the domain.
You’re prompted to enter the name of the domain you want to join.
Enter the name of the domain you want to join.
When you enter the domain name, the dialog box expands to allow you to enter domain credentials to allow you to join the domain.
Enter the name and password of a domain administrator account; then click OK.
You return to the Login Options page, which shows that you have successfully joined the domain.
Close the Users & Groups window.