1. A method for providing invitation codes, comprising: at a server system having one or more processors and memory storing one or more programs for execution by the one or more processors: receiving, from a client, an invitation-code request from a user of the client for an invitation code enabling a new user account to be registered, wherein the invitation-code request includes identifying information; determining whether the invitation-code request exceeds registration limit criteria based on the identifying information; in accordance with a determination that the invitation-code request exceeds the registration limit criteria, rejecting the invitation-code request; and in accordance with a determination that the invitation-code request does not exceed the registration limit criteria, providing, to the user, an invitation code enabling a new user account to be registered.