Hi, PFG.
Please, take a look at Options -> Users -> Change Permissions -> Create_topic , you can set it only for registered users.
You should also take a look at Options -> Users -> Manage Users & Groups, here you can create groups and insert user on it. Later you can set the permissions at "Change Permissions".
Options -> Application -> Extras & Add-ons -> Email & Notifications could be also useful for you.
Let me know if you have any issues.