When a group member is offline, incoming group messages are stored as offline messages for up to 7 days by default. The user receives these messages the next time they log in.
If you don’t configure this setting, the global Offline message storage duration will apply to group messages. If you do configure it, this setting will override the global one for group messages.
You can configure this on the Basic Features page in the Console, under Group offline message storage days.
If you have any questions about this feature, feel free to submit a ticket to us.