I don't have a firm number, but I would think that most players in silver could afford to join a guild. I have an alt account that is still stuck in bronze due to power requirements, but it still makes enough DEC that I could contribute 100 per week to a guild if I wanted to.

For me it's just a matter of whether you want to play in brawls. Otherwise I don't see a compelling reason to join a guild at this time, although I'm always hoping for new features to make them more important. Most new players can probably ignore them until they get to a point that a 50, 100, 200 DEC contribution every so often feels insignificant.