Pass data from a table row into a create login statement using dynamicSQL
I understand that the exercise is to use a cursor and dynamic SQL, but just to give you another view (and something to take beyond this one class I suppose), you can do a lot of administrative tasks like this without cursors. I feel that when you default to cursors for tasks where you do need to process things iteratively in a loop, you tend to then rely on them for tasks where you don't need a loop at all. So almost always better to think about what you need to do to a set of rows, rather than to each row. In this case, we need to execute the same set of commands against a set of users. Borrowing from the other answer:

DECLARE  @Administrators TABLE(firstname varchar(255),
                               lastname varchar(255));

INSERT INTO @Administrators VALUES('John','Wick'),
                                  ('Frodo','Baggins'),
                                  ('Gandalf','The Grey');

DECLARE @sql nvarchar(max) = N'';

;WITH u AS (SELECT u = QUOTENAME(firstname + lastname) FROM @Administrators)
SELECT @sql += CHAR(13) + CHAR(10)
  + N'CREATE LOGIN ' + u + N' WITH PASSWORD = ''temp'', CHECK_POLICY = OFF;
       ALTER LOGIN ' + u + N' WITH DEFAULT_DATABASE = MyGuitarShop;
       CREATE USER ' + u + N' FROM LOGIN ' + u + N';
       ALTER ROLE  OrderEntry ADD MEMBER ' + u + N';'
FROM u;

PRINT @sql;
--EXEC MyGuitarShop.sys.sp_executesql @sql;


Result:

CREATE LOGIN [JohnWick] WITH PASSWORD = 'temp', CHECK_POLICY = OFF;
  ALTER LOGIN [JohnWick] WITH DEFAULT_DATABASE = MyGuitarShop;
  CREATE USER [JohnWick] FROM LOGIN [JohnWick];
  ALTER ROLE OrderEntry ADD MEMBER [JohnWick];
CREATE LOGIN [FrodoBaggins] WITH PASSWORD = 'temp', CHECK_POLICY = OFF;
  ALTER LOGIN [FrodoBaggins] WITH DEFAULT_DATABASE = MyGuitarShop;
  CREATE USER [FrodoBaggins] FROM LOGIN [FrodoBaggins];
  ALTER ROLE OrderEntry ADD MEMBER [FrodoBaggins];
CREATE LOGIN [GandalfThe Grey] WITH PASSWORD = 'temp', CHECK_POLICY = OFF;
  ALTER LOGIN [GandalfThe Grey] WITH DEFAULT_DATABASE = MyGuitarShop;
  CREATE USER [GandalfThe Grey] FROM LOGIN [GandalfThe Grey];
  ALTER ROLE OrderEntry ADD MEMBER [GandalfThe Grey];