I see by the labels on your post that you're on Magento 2 Enterprise Edition. Are you by any chance using the new B2B feature set that's available for Magento Commerce users? If so, you may want to test out the Company Accounts feature set that's included. You can read up a bit on this functionality at: https://www.nchannel.com/blog/magento-2-b2b-multi-location-inventory/
Aside from those paid B2B features, I've had some luck with this extension: https://marketplace.magento.com/amasty-module-sales-reps-and-dealers.html
Another route might be creating an order attribute for an admin to fill out so that the sales rep is clearly defined when they place an order. This could be particularly helpful if you only want them to be associated with orders that they manually helped with, and not with orders that customers placed without them (ie. It's not their customer, they just help with orders on a sporadic basis). There are extensions that can help with order attributes as well, such as: https://marketplace.magento.com/amasty-orderattr.html
Once you have the data, and depending on extensions that you choose, it might take a bit of elbow grease to spit out a report, but that should be the easier part.
