Our latest update (2.2.0) has introduced a new authentification process to ensure secure access to new features.
One of the new features is multi-groups - see more about that here Multi-group feature
In order to allow access to your groups by all who were in them before, all members need to update their own app and go through the new authentification when they log in. Once that is done and they are 'verified' they will show up in all the groups they were in before.
Update directions can be found here Update Directions 2.2.0