Can I hook a form from a custom module? I have a form function called mymodule_join_form in MyModule and differentmodule_form_mymodule_join_form_alter in DifferentModule. It's not being called, though (I put `echo 'hello';die;)

I can get to it using form_alter with if (form_id == 'mymodule_join_form'), but shouldn't the first way work?
if it's a form alter you should take a look at http://api.drupal.org/api/drupal/modules!system!system.api.php/function/hook_form_alter/7

Also clear cache and look if you have enabled de módule.

Have you tray with a dpm('hello world') with devel module enabled?

oskar