according to https://developers.google.com/+/hangouts/running#running-private
google hangouts requires three permissions which are not asked for explicitly
by the unhangout installation. asking for these on the unhangout side will
smooth first-time users' experience of joining a hangout, since the permissions
will have already been granted, and will have no effect on existing platform
users, who have already granted all of these permissions upon logging in and
attending their first breakout session.
according to https://developers.google.com/+/hangouts/running#running-private google hangouts requires three permissions which are not asked for explicitly by the unhangout installation. asking for these on the unhangout side will smooth first-time users' experience of joining a hangout, since the permissions will have already been granted, and will have no effect on existing platform users, who have already granted all of these permissions upon logging in and attending their first breakout session.
tested in my installation, and works perfectly.