How to prevent notifications from a skype group?

type /alertsoff in skype groupchat.

There is a setting, although I've never used it so far (names might be off due to localization):

• Open the conversation/group you'd like to modify.
• Go to the Conversations menu.
• Find the Notification Settings....
• Here you can modify how/if/when to open the specific group or show messages as popups.

You can issue the following commands in a conversation that:

/alertsoff - Stops annoying popups but still keep you alive in a group
/alertson - Brings back popups

Much more commands in this link

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