Globally Subscribe?

Hi, I setup a forum to use for a focus group. This is my 1st try at doing a forum so I'm not very experienced so I need some help now that my searching has not been fruitful.

To keep everyone in the loop I would like to make it so all members are subscribed to all topics by default not by opt-in. Everyone has hectic schedules outside off the groups projects and people tend to forget to log-in to contribute so i want all the activity delivered to them which I assume will prompt more action.

Can this be done and how?
Thanks