With CollaborationGroupMember object, you can mass add users using API tools, such as: Data Loader. We discussed on how to add users to Chatter group in this blog.
By default, email setting for each user in a group will be Daily Digest, whether you create using API tool or add users manually in Salesforce.com
But, is there anyway to mass update email setting, let's to Weekly Digest, Every Post or even Turn Off?
YES, you can use the same object CollaborationGroupMember with API tool. When you insert users to a group with field: CollaborationGroupId (for Chatter Group) and MemberId (for User). There is another field NotificationFrequency in the same object, this field use to set email alert for each user in a Chatter group.
CollaborationGroupMember object is available in API version 19.0 and later. This field can only be set by the member or users with the “Modify All Data” permission. The valid values are:
• D - Daily
• W -Weekly
• N - Never
• P - On each post
Done, hope this blog help.