When searching initially for the above (title of my blog ^), I initially found no good apparent solutions. It was off to the drawing board!

LDAP – lightweight directory access protocol. Defines a syntax and tools for querying and enumerating access levels, groups and users associated with an active directory domain.

If you are a .Net developer or system admin on any system, you will likely encounter LDAP as well as other similar protocols and discover a learning curve with each.

See references below for other quick examples that may be helpful in the future.

ASP.Net (check if user has access to site based on AD group permissions):

public static string username {
        get {
            return HttpContext.Current.Request.ServerVariables["AUTH_USER"];
        }
        }
        public static string usergroup;
        public static string user_firstname;

        /// <summary>
        /// Check if current user browsing site is logged in.
        /// </summary>
        public static bool IsLoggedIn
        {
            get
            {
                //currently using AD

                if (HttpContext.Current.Session["IsLoggedIn"] != null)
                {
                    return (bool)HttpContext.Current.Session["IsLoggedIn"];
                }

                string loginname = ExtractUserName(username);

                DirectorySearcher search = new DirectorySearcher("LDAP://RUSSWHITNEY");
                //search.Filter = String.Format("(cn={0})", loginname);
                //search.Filter = String.Format("(&(cn={0})(SAMAccountName={1}))", "Direct Mail Admin", loginname);
                search.Filter = String.Format("(SAMAccountName={0})", loginname);
                //search.Filter = "(&(objectClass=group)(cn=" + "Direct Mail Admin" + "))";
                search.PropertiesToLoad.Add("memberOf");
                search.PropertiesToLoad.Add("givenName");
                System.Text.StringBuilder groupsList = new System.Text.StringBuilder();

                SearchResult result = search.FindOne();

                bool returnvalue = false;

                if (result.Properties["memberOf"].Count > 0)
                {
                    foreach (string prop in result.Properties["memberOf"])
                    {
                        if (prop.ToLower().Contains("cn=groupname one"))
                        {
                            utils.usergroup = "admin";

                            if (result.Properties["givenName"].Count > 0)
                            {
                                user_firstname = result.Properties["givenName"][0].ToString();
                            }


                            returnvalue = true;
                        }
                        else if (prop.ToLower().Contains("cn=groupname two"))
                        {
                            utils.usergroup = "user";

                            if (result.Properties["givenName"].Count > 0)
                            {
                                user_firstname = result.Properties["givenName"][0].ToString();
                            }

                            returnvalue = true;
                        }
                    }

                    //returnvalue = false;  //if empty or doesnt contain above stays as false from initialization
                }
                else
                {
                    returnvalue = false;
                }

                if (HttpContext.Current.Session["IsLoggedIn"] == null)
                {
                    HttpContext.Current.Session.Add("IsLoggedIn", returnvalue);
                }
                else
                {
                    HttpContext.Current.Session["IsLoggedIn"] = returnvalue;
                }
                return returnvalue;
            }
        }

        #region "Active Directory"

        /// <summary>
        /// 
        /// </summary>
        /// <param name="loginName"></param>
        /// <returns></returns>
        static bool IsExistInAD(string loginName)
        {
            string userName = ExtractUserName(loginName);
            DirectorySearcher search = new DirectorySearcher();
            search.Filter = String.Format("(SAMAccountName={0})", userName);
            search.PropertiesToLoad.Add("cn");
            SearchResult result = search.FindOne();

            if (result == null)
            {
                return false;
            }
            else
            {
                return true;
            }
        }

        /// <summary>
        /// 
        /// </summary>
        /// <param name="path"></param>
        /// <returns></returns>
        static string ExtractUserName(string path)
        {
            string[] userPath = path.Split(new char[] { '\\' });
            return userPath[userPath.Length - 1];
        }

        /// <summary>
        /// 
        /// </summary>
        /// <param name="userName"></param>
        /// <returns></returns>
        static string GetADUserGroups(string userName)
        {
            DirectorySearcher search = new DirectorySearcher();
            search.Filter = String.Format("(cn={0})", userName);
            search.PropertiesToLoad.Add("memberOf");
            System.Text.StringBuilder groupsList = new System.Text.StringBuilder();

            SearchResult result = search.FindOne();
            if (result != null)
            {
                int groupCount = result.Properties["memberOf"].Count;

                for (int counter = 0; counter < groupCount; counter++)
                {
                    groupsList.Append((string)result.Properties["memberOf"][counter]);
                    groupsList.Append("|");
                }
            }
            groupsList.Length -= 1; //remove the last '|' symbol

            return groupsList.ToString();
        }
        #endregion

Note:
Any snippets may have been condensed from their original sources for brevity. See references for original articles.

All server side code examples are in C# .Net.

References
Wikipedia, “LDAP”, http://en.wikipedia.org/wiki/LDAP
Wikipedia, AD, http://en.wikipedia.org/wiki/Active_Directory
StackOverflow, http://stackoverflow.com/questions/637486/how-to-get-the-current-users-active-directory-details-in-c
MSDN, http://msdn.microsoft.com/en-us/library/aa746475%28v=vs.85%29.aspx
CentOS, http://www.centos.org/docs/5/html/CDS/ag/8.0/Finding_Directory_Entries-Using_ldapsearch.html
Justskins, http://www.justskins.com/forums/active-directory-search-fails-84700.html
Weblogs, http://weblogs.asp.net/steveschofield/archive/2004/04/28/121857.aspx