How can I mute individual chats?

Android: To invoke the notification settings of a (single or group) chat, tap the bell icon in the title bar of said chat. The chat can be muted altogether or for a specified duration (and its notification sound can be adjusted, too).

iOS: If you prefer to not receive push notifications for incoming messages of a certain contact or group, open the contact’s or group’s detail view (by selecting the contact or group in the contact list). There, select “Notification settings”. Deactivate the option “Play notification sound” to receive silent push notifications, or set “Do Not Disturb” to “On” in order to not receive push notifications for this contact or group at all.

You will still receive messages in muted chats, but you will no longer be notified about them by push notification (meaning that you have to manually open the app in order to see whether any new messages were sent in muted chats).