You can mark all conversations as read within a specific group, or for all the groups where you're a member.
For a specific group:
- Go to the group home screen
- Tap the three dot menu, top right
- Tap 'Mark all as read'
For all the groups you are in:
- In the main menu, tap 'Groups'
- Tap the three dot menu , top right in the app, to the right of the 'Groups' heading on desktop
- Tap 'Mark all as read'