Instructions on how to do this (with ESXi 5.0) is available e.g. here in the VMware Online Documentation.
I first looked at AD integration when vSphere 4.1 was released and found one really annoying drawback in it that ruled it out from a possible implementation in our environment: When an ESXi 4.1 host is joined to a domain it will automatically (and repeatedly!) look up an AD group called "ESX Admins", and as soon as it finds this group it will grant this group Administrator permissions on the ESXi host. The real problem here is that the name ("ESX Admins") of this AD group is hard coded and can not be configured.
This may be a nice feature for small environments - you just need to create this group, fill in the necessary people and you are done. But if you think about an enterprise environment of a large company with lots of different sites, IT teams and vSphere installations, but only one Active Directory, you can not assume that all ESXi hosts in this company are managed by the same group of people.
When vSphere 5.0 was released I looked at the release notes and documentation to find out if this drawback was removed, but I did not find any positive information. Tests I did also showed that an ESXi 5.0 host behaves the same way, looks up the "ESX Admins" group and adds it with Administrator permissions.
However, recently I stumbled over the following when browsing the advanced configuration parameters of an ESXi 5.0 host:
|Configuring the "ESX Admins" group|
I searched for this again in the VMware documentation and the Knowledge Base, but did not find it being mentioned anywhere. So it looks like at the time this is completely undocumented, but it works as expected (I could not resist from immediately trying this out)!