I'd like to have my students debate each other on the topic of the week. So, I've split them in to 4 groups. How can I use the Groups function such that the two groups discussing a topic have a mutual forum (i.e. not individual group forums) but that the other two groups have their own mutual forum? In other words, I like for half the class to be in one forum, and the other half in another, but for that to be able to change and rotate.