REST API. Email notifications

If user's are not getting subscribed to messages they participate in:

This most liklely means they have not checked the "Automatically subscribe me to topics I participate in" checkbox on the "Subscriptions & Bookmarks" tab of the "my settings" page.  It's possible to default this setting to be on, but I believe you will need to contact support to get this turned on.

If no email is going out from the community:

Is this happening in a stage site or a production site?  I believe Lithium disables email on most stage sites so emails don't accidentally go out to people you don't mean them to go out to during testing.  We can enable email on your stage site to test email-related functionality -- please contact support to have email turned on for yoru stage site (and I would recommned you also contact them to have email turned off when you are finished so email doesn't go out to people you didn't mean it to go out to during testing).

If the issue is happnening on a production site, you should contact support to have them verify that mail is on.