If more than a single person is allowed to use a credit account they should be added as an authorized user.
To add an authorized user to an account you need to go to the credit account tab in the new/edit customer screen.
Select Account Type: Authorized User. from Customer>> Account Credit tab.
A dialogue will appear and you can search for and select the account this customer is allowed to use.
Click Authorize to Charge.
Then click on Save and Continue to save the changes which you have made.
Once you have saved, you will be able to make transactions, with the customer Credit account which you have authorized.