We decided that all users should receive daily Chatter digest via Email. Is it possible to turn this on via a mass update of all users, or do users have to turn it on themselves?
There is a Chatter email notification setting for each Chatter group. Members of a group can set this option, but it can also be set via the CollaborationGroupMember API. The default should be set to daily.
I don't think that is the daily digest you were referring to though, as there is another daily digest for records the user follows. I do not know if this can be changed through the API, but I think the default for this is also daily, so if a user doesn't have it set to daily, they would have intentionally changed it. Please let me know if you aren't seeing it as the default for a user.