Fast travel to "The Theater Settlement". Run around the streets (outside settlement) once, and look for a group of friendlies that has "En Route to Gather" above their heads when you aim at them. If you cannot find anyone, fast travel back to "The Theater" and run around the settlement again. Repeat this until you find a group that is on their way to gather something. Then, follow them to their destination. Wait for them to enter the location. One of the looters will have the option to press a button prompt. This will give them materials and unlock the "Resourceful Agent" achievement. You can also encounter groups of friendly gatherers randomly while exploring the world. By following them from the settlement, it is less random. If you cannot find anyone around the settlement, just keep playing normally and watch for friendlies running across the streets. Aim at them to check their status. If it says "En Route to Gather," follow them.