I signed up this morning with my phone number, and it didn't spam anyone (that I'm aware of).

The way it forms its 'secret' networks is similar to LinkedIn's degree system. Your phone contacts are your 'First Degree' connections, their contacts are your 'Second Degree' connections, etc.

Most posts from 2nd or 3rd tier connections never appear on your feed, it's only the popular ones that show up. It's interesting since you really have no idea who you're interacting with.

Disclaimer: I really have no idea how it all works, that's just my impression after a few hours of use.