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Manage your groups' delivery settings
Browse to https://groups.google.com
Under the Subscription column on the group you want to modify Delivery settings, change the default Each email setting to Digest, Abridged or No email
Managing other group members' Delivery settings
Open the group you want to modify in groups.google.com
Browse to People > Members in the left pane
Either update a single group member's Delivery setting by changing value under the Subscription column from Each email to Digest, Abridged or No email
Or, select multiple group members and click the Change subscription (envelope icon) to change multiple members' Delivery setting
