In case you do not want new members to read previous group messages, you can turn off the feature New member can see old messages.
To access the feature, you need to be the group owner or an admin, then follow these instructions:
Click the More (…) button at the top right corner of the group chat screen
Find the item Group settings.
Turn off or on the item New member can see old messages under Message settings.
While this permission is Off, new members will not receive old messages before the time they join the group.
While this permission is On, new members will receive old messages before the time they join the group. All members can access full history of Stored media, including photos, links, and files sent before the time they joined the group.
The group owner or admins will always have a full view of the message history and stored media for easier group management.
The following content is viewable to all members, even if permission to view the message and stored media history is disabled:
All content in the collections under Stored media, including the items that were added to the group before the time members joined the group.
All content in the group calendar and group board, such as reminders, polls, pinned messages and notes.