Join Avenue is a unique feature located on Route 20, accessible after you've progressed a bit in the story. It's a customizable shopping street that grows and develops based on your interactions with other players (via local wireless or Wi-Fi) and NPCs. Each person you interact with will open a shop on your Join Avenue, offering various services like item shops, nurseries, dojos for stat training, and beauty salons.
The more people you interact with, the more shops will open, and the higher their ranks will become. Higher-ranked shops offer better services and rarer items. You can also recommend shops to visitors, influencing what kind of shops appear. Managing your Join Avenue effectively can provide you with a steady supply of useful items, quick ways to train your Pokémon's stats, and even rare berries. It's a fun and rewarding way to engage with the game's social features.