How can I make a rate for regular guests (we have guests staying every week a couple of nights) for only internal use. They book by email and how can I choose this rate when making the booking by hand?

We have a lot of companies who reserve rooms and they are also want to have an invoice on there name. It would be great if there was a possibility to enter companyname/adres/VAT number etc. on the invoice and in the guestdatabase.