Rather than searching for specific users within the Zwift Mobile Link (ZML) and giving them one Ride-On, you can give 5 Ride-Ons to the riders in your proximity at once. You will need to use ZML to do this.
Once ZML is paired, and the map is showing, simply tap on your arrow icon to deliver 5 Ride-Ons to other riders close by. The Ride-Ons will go to people riding in either direction and to people participating in other sports (such as runners if you're cycling).
If you're in a group ride, the Ride-Ons will only appear to people within your group and category (so if you are in the 'B' category, your Ride-Ons will not go to people in the 'C' category, even if they are nearby).
So go for it! Give everyone around you some love with a Ride-On!