i hope i have made myself clear
Not entirely. How do you determine which groups need to have a pro-rated discount subtracted?

Is there anyway i can enter all these amounts then put the discount amount in a new cell at the end that will perform this action for me?
Enter the discount amount in one cell (say A1) and the divisor in another cell (say B1).
In C1 use this formula: =A1/B1

Then simply reference C1 in your "group cell" formula.
Example:
A15 formula: =SUM(A2:A10)-C1

Since your post lacks detail, the above may not be the solution you need. If it isn't, post a sample workbook and provide more clarity about what you need to accomplish. The workbook should contain an example of the desired results.