1. A method, comprising: receiving, by one or more server computers, an event definition and a user identifier from a first user device located at a first location, the event definition specifying an action, an event group, and an event location, wherein the event definition specifies that a device associated with the user identifier performs the action upon a condition that the device enters into or exits from the event location, the event group corresponding to a category of events that includes the action, the condition, and the event location, the user identifier being designated as a member of a group of one or more users that are associated with the event group; identifying, by the one or more server computers, one or more second user devices based on the event group, including determining that the one or more second user devices are associated with the user identifier; and synchronizing, by the one or more server computers, the first user device and one or more second user devices, the synchronizing including sharing the event group between the first user device and the one or more second user devices by pushing the event definition, including the condition and the event location, to the one or more second user devices, the pushing causing an entry into or exit from the event location by each of the one or more second user devices, as detected by the each second user device, to trigger the second user device that detected the entry or exit to perform the action, wherein the action is presenting a reminder in a visual, an audio, or a mechanical user interface.