Enhanced systems and methods for processing invoices, payments, and money transfers are described. One aspect of the invention is a computer-implemented method (and corresponding system and computer program product) for inviting an entity to open an account at a billing and payment system, the method comprising: receiving from a first entity an instruction to invite a second entity to open an account at the payment system, the first entity having an account with the payment system; receiving from the first entity a security question and an answer to the security question to accompany the invitation; transmitting to the second entity the invitation along with the security question; receiving from the second entity an acceptance of the invitation along with an answer to the security question; verifying the acceptance by comparing the answer received from the second entity to the answer received from the first entity; responsive to the two answers match, determining the acceptance verified and opening an account for the second entity; and creating a link between the accounts of the two entities for the purpose of sharing invoice information, making electronic payments, transmitting remittance information, and maintaining basic information about the invitor and invitee.