This will be a little labor intensive but it will work.
Created 18 running totals (1 per employer).
As an example here is Canden's.
Running total Name =Camden or CC
Field to summarize=amount billed field
Type of Summary=Sum (I assume a sum is what you need here)
Evaluate as "use a formula"
instr({Invoice.SalespersonID},"cc")>0
Reset as Never
and repeat this for each worker changing the initials per RT in the formula.
In your report Footer (running totals do not work in headers) Create 18 footers. Place a text box with each name and it's corresponding running total next to each name (one per footer). I recommend that because they are easier to move around in the future.
By grouping you will start excluding the "shared" jobs. This running total process will include them in each of the workers (e.g. counting it twice).
Note this will not preclude you from doing a grouping on the Salesperson ID and doing sums there where you could show the records once and each iteration of single and duoid' and an unduplicated real total.