Just stumbled upon this free module by Shape5. Only available for J15, sorry. Just wnated to let you know about it. It seems to do what you where asking for. Free downloads needs a free registration.

S5 CB Register Module - Joomla 1.5
The new CB Register module is pretty much a copy of the registration page that community builder uses. Having this page converted into a module allows you to put the registration page in a popup or similar lytebox window as we have done (click the register button in the upper right corner)...

(Anyway, in the long-run its better to keep things simple and original for easy CB updates! So, I would stick with the CB register page and maybe the lightbox link)

hello,
Thank you for this post.
I'm with J2.5.3, so I can not use it.
Finally, I put the registration page in an iframe, and I inserted into my homepage. it works just fine but I hope someday there will be a module to do it, it will be easier .
good day!

krileon wrote: We don't support wrappers, but at you can add &tmpl=component to the URL to just show the Joomla component content.

Hello,
This solution is good and I use it, but I have a question.
After registration, the page that opens contains all modules.
Is it possible to do the same thing as that (&tmpl=component)?

Thank you very mutch