WhatsApp group messages can be muted for 8 hours or one week or forever, why not just choose one of those?
Or better yet, just look in the app's notification settings in Android... one of the granular options is [all] "Group chats," and what you set there will take precedence anyway. "Default" with the notification sound set to None; or set to "Silent," with or without notifications appearing on the lock screen; or best yet, just go with Silent + Minimized = any such notifications are shrunk to a single (but expandable) line + display on the lock screen is disabled automatically.
i.e. still no need to open the full app to check for new messages -- just pull the notification shade down when you care to look, but otherwise they'll stay completely (and permanently) silent & out of sight.