At the moment, Jive does not support multi-forest, multi-domain setups. However, there might be some work-around options. If you'd like to create a case in your company's secure support space, one of our support engineers can take a look at your setup specifically and discuss your options.
I don't have any experience with integrating multiple AD forests, but perhaps a 3rd party solution like Okta may be an option: http://www.okta.com/what-we-do/active-directory-and-ldap-integration.html
We've had some success integrating this with Jive communities already and it's more flexible than OOB Jive.
Hi Jason - are you running some form of SSO ? If you are , there is a slick way of doing this! We have done this multiple times .