I'm already a Master Gardener and have done the first wave of jobs, but can't figure out how a friend can ask Streetpass people to "plant a seed"? Any idea of where to start would be helpful.
You have to talk to any Mii in your courtyard or in the shopping mall. You should have the option to plant a seed together.
According to Nintendo Wikia:
After gardening with the guests, they will hang out in either the courtyard or the shopping mall until new guests arrive. Players can ask them either what plant they are currently growing, or if they want to plant a seed together. When planting a seed together, the guest will dye the planted seed to his or her favorite color, which will cause the plant to bloom into that color.