It's just all of the offspring between two different species or hybrids (or a species and a hybrid).
A cultivar group is just different cultivars that share similar traits. If you had five different cultivars of Petunia with yellow flowers and variegated leaves, yet they were distinctive enough to be separate cultivars, that could be a cultivar group.
Right now, we have two schools of though. Technically, if you have a plant that's stable which happens to meet the description of a cultivar, then you can call it by the cultivar's name. There are plants that loom EXACTLY like D. 'Big Mouth' at Wal-mart, and technically you can call them D. 'Big Mouth'. Then there are people, like myself, that think that's stupid. In my opinion, if a plant is not genetically identical to the cultivar, then it is not that cultivar.
They want to make a grex registry for Nepenthes. I think it's great. Do it with Sarracenia, too. We'll be more respectable, like orchid growers. Some want to have an "Expert" panel, but I think that's a bad idea for several reason, not to mention pretentious. Profoundly so.