Mute it
Open Messages, tap the grid icon top right, open OSX Group Chat, then tap the chat name at the top. Switch Notifications off. You stay in the group, the alerts stop.
Leave it completely
Say the word in the support chat and we'll remove you.
Mute the OSX Group Chat with the Notifications toggle, or ask in the support chat to be removed.
Open Messages, tap the grid icon top right, open OSX Group Chat, then tap the chat name at the top. Switch Notifications off. You stay in the group, the alerts stop.
Say the word in the support chat and we'll remove you.