Intro: Soldering Brass to Stainless Steel Plus How-To Make Flux
This video shows how to soft-solder brass to stainless. The same technique will solder brass to brass (or copper, or any copper-alloy, or nickel, or steel, or iron).
Soldering steel/iron to steel/iron is a little trickier but works pretty well the same (just takes practice).
Stainless is usually a tricky metal to work with, but for soft soldering it's pretty easy to use. Just be sure to use an aggressive flux like I show in the video.