There are a variety of reasons why a consultant may need access to only some accounts within all clients a certain company handles. In this case, use the "Limited Consultant" role, found on the More and then Users page.
Click the wrench icon next to the consultant.
Click the "Limited Consultant" checkbox and save the user.
Now, use the "Switch Company" button in the top menu so see all your company's accounts.
Now go to the More option and Company Details in the client's account.
Select the consultant in the dropdown box and save.
Now, this consultant has access to the client's account, but he does not have access to other client's accounts. That's all there is to it!