A network system customizes communications and communication channels for delivery to candidate providers registering with the network system. Candidate providers progress through steps of a network registration process by providing input data at the various steps. The network system provides communications to request the input data and facilitate the candidate providers' progress. The network system can use a machine learning model to predict particular types of communications and communication channels (e.g., online messages, phone calls, physical mail, etc.) that are likely to be well-received by candidate providers. Thus, the network system is able to increase the expected number of candidate providers that successfully register to provide services to other users of the network system.