The info in this page is no longer accurate, and shouldn't be used as a guide until it is updated.
When running the DIY with Overlays turned on, the DIY will optionally include the OTClassListManager bundle in the overlay. Whether it is included or not is determined by whether the group_list_url parameter is included in the request. The group_list_url parameter should be a full url pointing to a location where an otml snippet is located or generated. The otml should be basic and contain one object: an OTObjectSet which encapsulates an OTObjectList of OTUserObject objects (and their possible parameters).
Inside the activity, you can ask for the OTClassListManager bundle, and call getUserList() to get the OTObjectList of OTUserObjects. You can then do as you wish with that list of users.
This is useful for providing an up-to-date, dynamic list of users in a class which can then be used for things like selecting workgroup partners, personalizing activities for the class, generating random pairings for discussion steps, etc.