How to setup an auto email for a specific group sign up?
Agree with Peter, you can find the CiviRules extension here: https://civicrm.org/extensions/civirules (or in the extensions list when you do Administer>Extension and then click on the Add New tab)

Documentation on how CiviRules work can be found here : https://docs.civicrm.org/civirules/en/latest/