I asked the friend that told me about that and he have told me that there is an option to deactivate notifications of a group for some hours, days or whatever... You continue receiving the messages of that group, but you won't get notifications for that group (you will continue other notifications from other groups or messages).

He does that (and I would like to) because some of my friends have a group were people sends about 100 messages a day and 200 on fridays and saturdays (some of them at hours were somebody of the group is already sleeping). Some of them have no battery for a certain hour at weekends XD.