Active directory groups should be of type "Security Group" to be added to SP.
You don't need to create separate SP Group if you have already created AD Group. All you need to do is add that AD Group as how you add a normal user.
For e.g. If you got a AD group called DOMAIN\Marketing-SharePoint, all you need to do is add that add that add group by going to Site Actions > site permission > grant permission and add that
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