Compouter smart group based on user AD group

Are you needing to scope policies to the computers based on AD group membership? If so you can use the limitations tab under scope to accomplish that. Basically, target the scope to a group that contains all the computers in question(or potentially target all computers, though that option comes with some risk and should be considered thoroughly before using it), then under Limitations add the LDAP user groups.

If the smart group itself is your goal, then I don't know of a way to accomplish that exactly. It is possible to create smart groups based on AD attributes, you just need to create a computer extension attribute to pull the AD attribute.