There are two major approaches you can take here.
1) You can have the shell give the modules a reference to each other so that they can call methods on each other (or just one-way; the new module might know the login module, but not visa-versa). This approach will be most loosely coupled if you use interfaces to interact with rather than references to the module classes themselves.
2) You can 'plumb' it to the login module using the pipes utility. A pipe allows you to send messages to one or more modules connected to the pipe without having a reference to those modules.
If you aren't familiar with the Pipes utility, you can get a quick overview of it here:http://puremvc.tv/#P002/
And download it here:http://trac.puremvc.org/Utility_AS3_MultiCore_Pipes