Symfony uses the schema.yml to create a module for every db table. So consider the users table can quickly be modified in the users module with a simple example:
$users = new users;
$id = $users->getUserId();
That I get. However, what if I am in another module with its own db table? How can I access user data from there?
Side question: Is there a good Mac IDE to find all the variables accessible from a certain module? I am picking up a project, and am a bit confused with structuring and variable availability/passing. Any tools/tips?
The schema.yml is used to generate your MODEL objects, not modules. Your model defines your project's underlying data structure, and your modules group together actions and views.
It sounds like you're using Symfony generated admin modules, is this correct? If so, please be more specific as to where you want to make use of the different models. Is it in the list? The edit form? etc..
Generally speaking, your model objects are accessible from anywhere within your project - they are not limited to a specific module or even an app (frontend, backend, for example). So it's possible to access many models from within one action. In your case you could try Doctrine::getTable('Users')->find(USER_ID_HERE) which would fetch a specific Users object.
Your example code has an error in it. It should be $users = new users();
Once fixed, that code should work anywhere from within your application.
As for IDEs, check out Netbeans. Its auto-completion and code reflection is pretty good.