Well, quick and dirty uses the following components:
DATEDIFF(dd, [fromdate], [todate]) + 1 - for the total number of days (leave out the +1 if you don’t want days inclusive)
DATEDIFF(wk, [fromdate], [todate]) * 2 - for the number of weekend days in the most common cases
CASE WHEN DATEPART(dw, [fromdate]) = 1 THEN 1 ELSE 0 END - to subtract if starting on Sunday
CASE WHEN DATEPART(dw, [todate]) = 7 THEN 1 ELSE 0 END - to subtract if ending on Saturday
Line 1 with all the others subtracted should (I think) give you a weekday count between the two dates, but in any case playing around with those components should get you most of the way there if not.
Most more reliable solutions involve keeping a date table maintained, which we’ve found well worth while but may not be for everybody.