How do you import Active Directory security groups for SP into SharePoint itself?
I have created the group and added the users, just want to have that group displayed when you go to the Site Permissions for a particular site
Add the AD group to one of the built in SharePoint groups like SiteName Owners or SiteName Visitors.
In addition to Robert's suggestion, you can also grant an AD group permissions (Full Control, Read etc) directly instead of adding them to an existing SharePoint group).
Might be worth noting that with SharePoint you have to "grant access" to a group as opposed to "import" a group, in order for them to show up on the site permissions page.