当Person or Group字段允许多人时，通过sdk对象读取用户信息

 SPFieldUserValueCollection values = (SPFieldUserValueCollection)item["Users"];
string initUsers = "";
for (int i = 0; i < values.Count; i++)
    initUsers += (initUsers.Length > 0 ? "," : "") + values[i].User.LoginName;
    txUsers.CommaSeparatedAccounts = initUsers;
}