There are a few!
I would try B2B/Wholesale Solution first.
I’ve used BOLD Custom Pricing: Wholesale for some client websites but BEWARE.
BOLD works pretty well when you have it all set up but if you decide it’s not for you, you have to uninstall it through the app and not just from your Shopify apps page in the admin. If you uninstall BOLD incorrectly, it can leave you with an array of issues from the leftover code.
Both of the above-mentioned apps work by simply adding a tag to a customer.
And of course, if you’re doing enough revenue, Shopify PLUS could be worth it for you. Wholesaling features are built right into that plan but it does start at US$2000 per month.