1. A method, comprising: processing requests from a plurality of users to join a group event, the group event defined for an activity metric for each of the plurality of users for a period of time; processing activity data 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; generating a message, wherein generating the message includes, identifying, based on the identified change in relationship, 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.