When adding users to a group and giving them a group e-mail address like group@abc.def it seems to be impossible to add this address as a "send as" address for each of the users. It can be added, but the confirmation e-mail will not be delivered.

In order to work around this issue, the group can be temporarily deleted. One user can be assigned the group address. Having done this, all group users can be given the "send as" e-mail address and the confirmation e-mails arrive. Having set this up, the group e-mail address is deleted for the single user and the group is re-created.