After Safaricom issues you a paybill, the paybill owner should get access to an 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 preparing a letter (addressed to Safaricom) 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