StreetPass Miis arriving by invitation

On the Nintendo 3DS system players can collect other Miis via StreetPass and then visit and view them in the StreetPass Mii Plaza. But it turns out that many more Miis will show up in the plaza and they arrive by other means... perhaps by way of SpotPass (local wi-fi), though on this I am not sure.

What I'm curious to know is what are the ways for Miis to arrive at StreetPass Mii Plaza by Invitation?

These are the ones without any hats or costumes way in the back, and when you select on them they give no information except for a message - "Met via Invitation."

They don't show up at the gate to meet your Mii either.

Solution 1:

According to GameFaqs:

If you have the option turned on, and play Mario Kart 7 online every person you race will get in your Mii Plaza. Also any person that you tag in Super Mario 3D Land will be there too. You also get them in Sonic Generations.

They add to your Mii Plaza for the purpose of Achievements.

In other words, you get those Miis by playing online with other people by certain games.