How can I mute a post group or group chat?

Select the specific post group / group chat in the main item "Groups" or "Chats". Click on the profile picture or the name of the chat/group at the top.

Here you can select the option "Mute chat/group".

Now you will not receive any push notifications on your smartphone/computer about activities and news in this group or chat, but you will still see them in the app by means of the red dot.