I think Sales Reps is probably what you're going for, but maybe you have some more specific requirements that won't work for that. You have to assign people as sales reps on their employee profile, then make sure their role has the Sales Role checkbox checked, and then add them as the sales rep to that customer. It should restrict access to those customers based on the supervisor hierarchy set on their employee record (e.g. if I'm your manager I can see all your customers as well as mine). Give it a shot and see if that's what you're after.