1. A method, comprising: processing requests from a plurality of users to join a group event, the requests being received over a network from a plurality of client devices, the group event defined for an activity metric for each of the plurality of users for a period of time; processing activity data, received over the network, associated with each of the plurality of users during the period of time to determine and track the activity metric for each of the plurality of users, the activity data for a given user being determined from motion data detected by an activity monitoring device associated with the given user; analyzing the activity metrics of the plurality of users to determine a ranking of the plurality of users based on their activity metrics, and to identify a change in relationship between activity metrics of two or more of the plurality of users, wherein the change in relationship is defined by a passing event wherein an activity metric of a first one of the users surpasses an activity metric of a second one of the users, wherein the change in relationship is further defined by a difference between the activity metrics of the first one and second one of the users that exceeds a predefined amount; generating a message, wherein generating the message includes, in response to identifying the change in relationship, identifying one or more user accounts of select ones of the plurality of users to receive the message, selecting a message template based on the identified change in relationship, and populating the message template to generate a message describing the change in relationship; sending the message to the one or more user accounts, wherein sending the message to the one or more user accounts includes sending the message over the network to one or more of the client devices for display to the select ones of the plurality of users.