I'm not sure I understand which will be the problem with testing.

Here's another idea: We can add a `created_on` entry on `GroupMembership` storing the date the object was created. Again following the same principle, if `GroupMembership.created_on` > (1 Day | last_send_date) send out emails.

This way we also store something potentially useful about `GroupMembership` and we can also send out emails.

How does this sound?