What's the best way to make sure all the members are getting notifications?

I'm basically trying to keep people informed of new activity. I notice I didn't get an email notification from a friend who posted a comment to her blog after I posted a comment. So I am still confused about how to best set things up so that all new members (1) see activities and (2) get notifications from all their friends and groups by default. 

Otherwise people won't come back to check for messages!

It's fine if people want to go in and turn off their notifications afterwards, but I would like there to be maximum notifications by default. In the past, my experience is if that people don't get notified they don't know where to look for new activity. That's why I'd also like the "activity widget" to default to "friends" and not "mine." It would be nice if that also included "my groups."

doug