I'm still not sure what the problem was but after conferring with Ximena and Jason I decided to e-mail a link to the group directly to students. In class on Wednesday, I wrote the URL on the board for anyone who hadn't checked their e-mail, letting them know to e-mail me if they have any trouble. So far, it seems to be working since most of the class has now joined the group and I haven't gotten any e-mails.
I gave both classes an online assignment to complete over the weekend and asked them to read each other's posts before class on Monday. We'll see how that goes. It was a bit of a shaky start but having a Plan B (and C) definitely helped and I didn't lose too much time. Let's hope next week goes a bit more smoothly!