I played the game without streetpassing anyone who owned it. Funny thing is I didn't really need to.

Once every 24 hours or so you can go to the trader and "Update Data" and "Add Friends" over the internet which allows you to receive random data from the internet. Each person adds a population of 1 to the town, and allows you to summon them in battle once.

I ended up having 32 people in my towns population which was encough to max out everything by chapter 6.