Hi,
You have two ways to subscribe your joomla users to your lists.
The first option is to enable Joomla integration in Acymailing->Configuration->Subscription and it's documented here: https://docs.acymailing.com/setup/configuration/subscription#user-integration
Once you enable this new users will be subscribed to the lists specified. To add your existing Joomla users you need to import them once. Go to Acymailing->Users and choose import and then Joomla users.
The second option is to setup Automations (you need the Enterprise version for this). Using Automations you also can assign users from specific Joomla users groups to specific Acymailing lists. Automations are documented here: https://docs.acymailing.com/main-pages/automations