Grouping data by MONTH on DATETIME column in SQL Server
If you're just looking for October regardless of the year, then the easy way would be to just COUNT the records from that month:

SELECT COUNT(*) AS NumberOfJoiners FROM Employee WHERE MONTH(DateOfJoining) = 10;


or

SELECT COUNT(*) AS NumberOfJoiners FROM Employee WHERE DATEPART(MONTH, DateOfJoining) = 10;


If you want to group by year, then you'll need a group by clause, otherwise October 2013, 2014, 2015 etc would just get grouped into one row:

SELECT 
  DATEPART(YEAR, DateOfJoining) AS YearOf Joining, 
  COUNT(*) AS NumberOfJoiners 
FROM Employee
WHERE DATEPART(MONTH, DateOfJoining) = 10
GROUP BY DATEPART(YEAR, DateOfJoining);