There isn't a daily digest.
I've attached a screenshot from the usergroup settings. You can automatically assign forum subscriptions to groups
when a member is assigned to a group. So if you want to subscribe your current members you will need to run an sql query. As far as a flood of emails, it really depends on if your server can handle it or not. I know if I subscribed all of my members there would be a revolt

, but I think my server could handle it.