You can mute individual or group chat notifications for a specified period of time. You'll still receive messages sent to the individual or group chat, but your phone won't vibrate or make a sound when they're received.
Alternatively, open the individual or group chat. Press Options > View contact or Group info > Mute notifications. Select the length of time you'd like to mute notifications, then press OK.
Alternatively, open the individual or group chat. Press Options > View contact or Group info > Mute notifications > None.