Excel If code for using the alphabet

I am not a type of person that comes to forum for every little problem, but I have found something that I just cannot figure out.

I am creating a workbook for my co worker to help them be more productive.

We have thousands of clients, we distribute the clientele out by their name; for example if the clients name is between M. Coburn thur A. Duran client will go to case worker Terra Jones. I need a code for when a case worker types in the client’s name it will show which case worker the client belongs to. This picture will show you kinda of what I need:

EXCEL1.JPG

Thanks!