I have an account on Google+ but Android Market does not allow me to download Google+ application.
Well I couldn't download it from the market app on my phone in Germany, but I was able to install it from Play Store.
Otherwise you might try installing market enabler and fake that you are in the US.
If you don't mind getting it from untrusted sources, you can google "google plus android apk", and this will show you a list of repositories and forums to download it from.
Note that, as a is the nature of such, let's call them, untrusted sources, there's no guarantee that either they or the google query will persist for long, so I'm not even bothering to link to anybody.
You can install from here but may be its in development phase and not activated