I have the Soundtouch 300 soundbar and 500 bass module, and 2 Soundtouch 10 speakers already set up in my house and the system works beautifully.
I am trying to add a 3rd Soundtouch 10 speaker. I select the same network and password the other pieces are on, and it continually tries to create it's own network and give me an error and says can't connect. Well, of course you can't connect, because you created a network that is not mine and no other components are on that.
I found the solution. I unplugged all of the other soundtouch pieces. Not just turned them off, but actually unplugged them. Then I only plugged in the new Soundtouch 10 speaker and went thru the process of adding it, and this time it worked. After the new speaker was successfully on my network, I then went and plugged the other pieces all back in and they went thru the bootup process and hooked. Then I simply just turned them all on by pressing the Plus sign (within the Soundtouch program). All is working now.