Open vSphere Client and login with the root username and password. Once it opens..
Click the configuration tab.
Click DNS and Routing -> Properties
Make sure that you have the Host Identification Name & Domain Filled in. Next make sure you have the correct Preferred DNS server listed.
Click Authentication Services -> Properties
Click the drop down box and select Active Directory.
Type your domain in the Domain: section. Click Join domain and enter your AD credentials.
Click ok to close the Directory Ervices Configuration box.
Now open your AD server and navigate to AD Users and Computers.
It doesn't matter what OU you put it in regardless of what the video says below. I put it in an OU I created and it worked fine.
Inside that OU create a new Group and name it what ever you want. I named mine ESX Admins and ESX Users. Add some people in those group and then jump back over to vSphere.
Click the Permissions Tab
Right click and select Add Permissions
On the right side, select the permissions you want to assign. I selected Read-Only for the ESX Users and Administrator for ESX Admins.
Click Add and select your domain from the drop down box at the top. Scroll down to your group and click add. Click Ok -> Ok.
Log out and try logging in. Make sure you log in like domain\username or username@domain
Sources: Link Link
For this to work after you update your active directory password, you must also follow the instructions I wrote up here.