Groups: from preview to join

I have started a new Facebook group, but the structure seems to have changed. I invited members, and some accepted the invitation, and I approved them. But they are not showing up as members. It seems that Facebook has changed the way its groups work - now a member accepts an invitation, previews the group, and decides whether to join. How do they join the group? How do I tell them that they are not members yet and NEED to join the group? It's not the way it was the last time I created a group!