Only members of a club can opt-in to receive app notifications about a new club event. However, some clubs have a mix of private events (for dues-paying members) and public events (for the general public). It makes sense for members of the general public to receive app notifications about public events for a specific club. This might be called "following" the club (as opposed to joining it). This allows runners who would like to attend public events but are not interested in joining the club to receive the same notifications that club members get about public events.