what you need is a plugin that will impart SSO functionality; or if you r not too keen on SSO, u could look in to delagated authentication mechanism in Jive
There is no generic sso plugin available. All the SSO plugins available rght now are customer specific. You can refer to http://docs.jivesoftware.com/clearspace_community/latest/ExampleAuthenticationandAuthorization.html on how to create an SSO plugin.
Here is another document on it with some comments from others at the bottom who have done this.
If you wish to log someone in from another page, you want to have access to certain java objects. If you have a populated user object, you can call this.
User user = userManager.getUser(usernameFromCookie);
JiveUserAuthentication auth = new JiveUserAuthentication(user);
If you are creating the user for a first time login, this may be helpful
UserTemplate ut = new UserTemplate();
user = getUserManager().createApplicationUser(ut);
}catch (com.jivesoftware.base.UserAlreadyExistsException uaee)
log.info("User already exists " + uaee);
catch (com.jivesoftware.base.EmailAlreadyExistsException eaee)
log.info("email already exists " + eaee);
JiveAuthentication auth = new JiveUserAuthentication(user);
May be kind of late and you already have a solution, but here is another idea that I recall from digging into the remember me functionality in Spring Security and how it relates to Jive.
If you pass in the username and password in the request object from the other application into the jive page, it might work out of the box. I haven't tried it, but I have dug into the remember me filter and that seemed what it was looking for.