collection of tidbits I've pulled together

ActiveDirectoryGroupFinder.cs

    public class ActiveDirectoryGroupFinder
    {
        public string GetPrimaryGroup(DirectoryEntry aEntry)
        {
            var primaryGroupID = (int)aEntry.Properties["primaryGroupID"].Value;
            var objectSid = (byte[])aEntry.Properties["objectSid"].Value;
            var escapedGroupSid = new StringBuilder();
            // Copy over everything but the last four bytes(sub-authority) 
            // Doing so gives us the RID of the domain 
            for (uint i = 0; i < objectSid.Length - 4; i++)
            {
                escapedGroupSid.AppendFormat("\\{0:x2}", objectSid[i]);
            }
            //Add the primaryGroupID to the escape string to build the 
            // SID of the primaryGroup 
            for (uint i = 0; i < 4; i++)
            {
                escapedGroupSid.AppendFormat("\\{0:x2}",
                                             (primaryGroupID & 0xFF));
                primaryGroupID >>= 8;
            }
            
            var searcher = new DirectorySearcher
            {
                Filter = "(&(objectCategory=Group)(objectSID=" +
                         escapedGroupSid + "))"
            };
            searcher.PropertiesToLoad.Add("cn");
            return
                searcher.FindOne().Properties["cn"][0].ToString();
        }

        public IList<string> GetGroups(string lDAPName, IList<string> foundCollection)
        {
            using (var directoryEntry = new DirectoryEntry(lDAPName))
            {
                var groupsIter = directoryEntry.Properties["memberOf"].GetEnumerator();
                while (groupsIter.MoveNext())
                {
                    if (groupsIter.Current == null) continue;
                    var groupLDAPName = groupsIter.Current.ToString();
                    using (var groupDirectoryEntry = new DirectoryEntry("LDAP://" + groupLDAPName))
                    {
                        using (var groupSearcher = new DirectorySearcher(groupDirectoryEntry))
                        {
                            groupSearcher.SearchScope = SearchScope.Base;
                            groupSearcher.PropertiesToLoad.Add("cn");
                            var groupResult = groupSearcher.FindOne();
                            var groupName = groupResult.Properties["cn"][0].ToString();
                            if (foundCollection.Contains(groupName)) continue;
                            foundCollection.Add(groupName);
                        }
                    }
                    GetGroups(String.Format("LDAP://{0}", groupLDAPName), foundCollection);

                }
            }
            return foundCollection;

        }
    }