You can visit other players' farms or invite them to your farm to lend a helping hand. When you invite people, you'll need to prepare souvenirs (one for each visitor), and when you visit another player's farm, you'll need to choose a souvenir to bring. It's always good to be gracious, whether as a host or visitor.
In Multiplayer mode, you can chat with others and help each other out with farm work. While you're visiting a farm or hosting other players on your farm, time won't progress, the weather will always be sunny, you won't lose stamina or health, and you can't leave the farm without quitting Multiplayer mode.
In Multiplayer mode, you can help other players with farm work by waving a magic wand, which helps their crops and animals prosper. You can also communicate with other players via messages and emotes.
Once you've activated StreetPass, you'll be able to meet all kinds of new people! When you pass by other Nintendo 3DS systems that have StreetPass enabled for this game, you'll automatically exchange profiles, and they may show up to visit your town or Trade Depot as NPCs.