As you leave a settlement or control point, you'll often see four folks heading out to get supplies. It'll say over their head "En route gathering supplies" or words to that effect. All you need to do is follow them.
Here's an example of some folks leaving a Control Point:
Full size https://imgur.com/a/7FxF98V
Eventually, they will get to a place to get their supplies, move to the person gathering the supplies and press X to give them some of your supplies.