Referring to the process 300, the user list generation server can receive an original set of user identifiers for an original set of users that performed actions at an electronic resource (e.g., website, webpage, application, etc.) of an entity (e.g., digital component provider or publisher) (302). The original set of user identifiers can include users who perform actions on websites of digital component providers. For example, the original set of users can include users that select certain products, add those products to online shopping carts, and/or purchase those products. The original set of users can also include users that perform other actions on web pages or applications, such as clicking on links, searching, and spending certain amounts of time on one or more web pages. The original set of user identifiers can be a user list for a user action group that includes users that each performed one or more particular actions at one or more electronic resources of a publisher or digital component provider.

The user list generation server populates a user list with the original set of user identifiers in 304. As described throughout this document, the user list generation server can continue to update the user list by adding user identifiers to the user list. The user list can be updated at predetermined times. The user list can also be updated randomly at one or more different times. In some implementations, the user list can be continuously updated using the techniques described herein. The user list can be for a user action group that includes users that have performed the same or similar actions at resources (e.g., web pages), or for another type of user group. The user list can be expanded to include more users who share similar interests and are likely to perform the some action in response to receiving a digital component.