Ethically yes, but technologically no. It's possible to create a
Google (or Yahoo or MSN) group and add people without their knowledge.
It is also possible to set the groups up as broadcast only, so only
you use them and they handle distribution. When the people receive
the messages though they'll notice the group's address in the to field
instead of their own, so they'll know that something is up.
So ethically, yes you have to let them know, but it is possible to do
without their knowledge.