I've done several startups, and always managed to attract several people to work for free for an extended period of time while the idea was getting off the ground.
I usually work out an elevator pitch, and then literally tell everyone I know about what I'm thinking of doing, like this:
"How are you, anything new?"
"I'm great; in fact I'm working on a new startup, let me tell you about it"
My friends enjoy hearing what I'm up to, and it usually doesn't go beyond a social interaction. But I'm always surprised that maybe 1 in 50 people I talk to say that they find this interesting and ask if they can get involved. That's another person on my team.