You could create Security Roles for each group and set the roles as a Public Role. Your users can then manage their subscriptions to one or more roles on the Manage Services section of the User Account module (e.g. on their user page). This gives you the Newsletter functional you require and means users can subscribe or unsubscribe at any time. You could also use the RSVP Code and RSVP link options and include this information in the Welcome email to push the information to them.