So stated differently:

Person gets a prorated share of the hours, from the sum of the hours listed in the three middle columns, if his name in in one of the right hand columns. Prorated share to be contingent on how many others share the work for that task.

Edited next:
You need additional columns, one for each individual.
In that column generate a formula that calculates the hours that person receives based on the sum of hours divided by the number of workers sharing that task, but only if he is one of the workers for that task.
Copy formula across and down. Formula should reference name at top of column when comparing against names to left. That way the formula is generic and works for all cells in new columns.

sum workers total at bottom of new columns.




Isn't this what ms-project was created for?