Yes, there's a way to view monthly paid invoices by sales rep in QuickBooks Online (QBO), @Mick. I'll gladly share more on how you can do this below. 

If you're using the QBO Plus version, you can track your sales by sales rep using either the Class or Location tracking features. You need to create your Class or Location list and add your salespeople to track their sales individually. Then, you'll have to select the salesperson associated when creating your transactions. 

After that, you can pull up sales reports by Class or Location QuickReport and customize them to only display your paid invoices on a specific month. You can learn more about the process by checking out this article: Track sales in QuickBooks Online.

If you're using the Advanced version of QBO, you can create a custom report by sales rep. Please see this article for the complete guide: Create custom reports in QuickBooks Online Advanced.

Also, QuickBooks gives you a variety of financial reports that tell you about the different aspects of your business. You may want to check out this article for an overview of how to run basic ones and what you can do more with them in QBO: Run reports in QuickBooks Online.

Let me know in the comments if you have other concerns about managing invoices by sales rep in QBO. I'm always ready to help. Take care, @Mick.