Counting Rows (SQL Server 2008 R2)

I have a tablix with a textbox containing a lookup expression.

=Lookup(Fields!Last_Assigned_To_ID.Value, Fields!Internal_User_ID.Value, Fields!Skill_Group_Name.Value, "Users")

This will either display a blank or 'Manager (FO)' skill group. 

I want to count the rows with 'Manager (FO)' in this column and another count when blank.

So lets say my tablix display the following data

Skill Group Name

(Blank)
(Blank)
Manager (FO)
Manager (FO)
(Blank)

The textbox for the count of Manager (FO) should display 2
The textbox for the count of blank skill group should display 3

I can't figure out how to use the Count(IIf.... I found in one of the threads here. 

These are my datasets:

WorkItems:
-Last_Assigned_To_ID

Users:
-Internal_User_ID
-Skill_Group_Name

Any help is appreciated.  Thank you