A machine may be configured to subscribe a user to a data feed that corresponds to an activity. The machine may generate an optically scannable code that encodes an activity identifier of the activity. The machine may cause a first device to access this optically scannable code, and the first device may present the optically scannable code to a second device. The second device may scan the optically scannable code, and the machine may detect that the optically scannable code has been scanned by the second device. In response to this detection, the machine may subscribe a user of the second device to a data feed that corresponds to the activity. Hence, a first user may present the optically scannable code using one device, and a second user can scan the optically scannable code using another device and be subscribed to the data feed for the activity.