I've moved this post over to our Server admin section. I recommend looking around here to see if there is anything that can help you out. I believe Toby Erkson is really knowledgable about groups.
In Groups click the 'Import' link:
Type in your AD group in the 'Search' box then click the 'Search' button:
The AD group(s) will show up in the 'Select' box. Click the one you want. If what you're looking for doesn't show up and you know you are spelling it correctly then try prefixing it with the AD domain and a backslash e.g., domain\AD_Group.