The "Community Builder Plugin Framework API Documentation" is now uploaded into developer.joomla.org, accessible to all loged-in users here:
Community Builder Plugin Framework API Documentation (33 pages, 1.1 Megabyte PDF file)
Please take the time to read through it and also to look into the existing RC2 plugins (3PD developers have access to RC2 and some CB team resources) before starting development.
We will link to it from this site soon as well as setup a public space for 3PD plugin developers, in addition of the private forums they already have at the cb team private website.
3PD developers are encouraged to contact Nick (nant) by PM to coordinate PR and integration, and myself for integration coordination and code reviews.