After Safaricom issues you a paybill, the paybill owner should get access to Mpesa portal for that paybill, however, if that is not the case, then you will have to ask Safaricom to create for you a Business Admin for that paybill.
You can do that by emailing Mpesa Business with the below details on a company letter head, signed and stamped by top management.
Organization Short code :
ID type (national ID, Passport):
Administrators User Name:
Once you have this ready, attach it with the id of the business admin and email it to : email@example.com
Have further questions, email us: firstname.lastname@example.org