Actually, at https://groups.google.com/ I can answer a mail with the group name with no problems. But, if I try to do it from Gmail there is no option to answer the email with the group email even if the user has the permission to do so. Yes, I could use aliases, but the problem with that is every member has to add/remove the alias manually and I want something automatic.
I want every member of the group to have the ability to answer emails with that group email once they are added to the group.