Google has just unveiled their plans for group waves. The Google Wave Blog outlines the steps to set up a Google Group (yes, you have to use another tool) then add the group as a contact in Google Wave.

The service does not yet allow you to add users by their account ( only), and the permissions can only be handled through the Google Groups interface. It also seems that to avoid spam (being able to send messages to an everyone in a public group at once) users must pro-actively seek out the group wave and follow it to move it to their inbox [currently waiting for confirmation of this].

Like any Wave feature, this is still being developed and its behaviour could change over time. The Wave team have said they will continue to enhance the groups feature to make groups easier to navigate.

Waving with groups - Google Wave Blog.