Convert Date/Time to Date

You could do it like this in MSSQL

SELECT COUNT(RSO_ParentID), AssignedDateTime FROM Task WHERE OwnerTeam='2nd Line Support' AND AssignedDateTime>=DATEADD(D,-7, GETDATE()) GROUP BY AssignedDateTime 

Note: This question was not tagged SQL Server 2005 when I answered it. I am leaving the answer, because it is appropriate for SQL Server 2008+.

If you just care about the date and not the time, you need a combination of casting and datediff():

SELECT COUNT(RSO_ParentID), AssignedDateTime FROM Task WHERE OwnerTeam = '2nd Line Support' AND AssignedDateTime >= dateadiff(day, -7, cast(getdate() as date)) GROUP BY AssignedDateTime; 

Note that you can also express this using functions on AssignedDateTime. That is generally a bad idea because it often prevents the use of indexes for the query.

I also am guessing that you want the results by day:

SELECT COUNT(RSO_ParentID), cast(AssignedDateTime as date) FROM Task WHERE OwnerTeam = '2nd Line Support' AND AssignedDateTime >= dateadiff(day, -7, cast(getdate() as date)) GROUP BY cast(AssignedDateTime as date); 

or a total, in which you don't want a group by clause:

SELECT COUNT(RSO_ParentID) FROM Task WHERE OwnerTeam = '2nd Line Support' AND AssignedDateTime >= dateadiff(day, -7, cast(getdate() as date)); 

If you are using MSSQL, use DATEDIFF.

SELECT COUNT(RSO_ParentID), AssignedDateTime FROM Task WHERE OwnerTeam='2nd Line Support' AND DATEDIFF(DAY,AssignedDateTime,GETDATE()) <=7 GROUP BY AssignedDateTime 

Try this, it will check only date part, but not time part

SELECT COUNT(RSO_ParentID), AssignedDateTime FROM Task WHERE OwnerTeam='2nd Line Support' AND CAST(AssignedDateTime AS DATE) >= DATEADD(D,-7, GETDATE()) GROUP BY AssignedDateTime