收集用户或用户组栏里面所有的用户和用户组 ??

 List<SPUser> Users = new List<SPUser>();
            List<SPGroup> Groups = new List<SPGroup>();

            using (SPSite Site = new SPSite("http://windbell"))
            {
                SPWeb Web = Site.RootWeb;
                SPList List = Web.Lists["测试列表"];
                SPItem Item = List.Items[0];

                SPFieldUserValueCollection Values = (SPFieldUserValueCollection)Item["用户和用户组"];

                foreach (SPFieldUserValue Value in Values)
                {
                    if (User != null)
                    {
                        SPUser User = Value.User;
                        Users.Add(User);
                    }
                    else
                    {
                        SPGroup Group = Web.Groups.GetByID(Value.LookupId);
                        Groups.Add(Group);
                        Users.AddRange(Group.Users);
                    }
                }
            }