roles and active directory

i have a web app that is using active directory for authentication...so instead of setting up my roles in the config file like I normally would, they were set up in the security groups of the active directory.  How can I programitcally access these security groups?  i would normally use something like if user.isinroles("admin")...can I still use isInRoles and if so what should I use as my argument, i tried putting in the security group but that didnt seem to work?  also would that be the best way of doing it or is there another way to access the security groups?   thanks in advance. 
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