Good question. From what I've been able to determine, they are not the same thing on the level that they exist. OBDude mentioned Michael Newton. I'm a trained regression hypnotherapist. I really like Newton's books, but he is filtering the information through his personal biases. In other words, the truth comes to us only in the way we can accept it at the time, based on our beliefs. Hypnotherapy can be done in many ways, and some styles use leading questions which have the effect of steering the client's answers in a certain direction. No hypnotheapist or client picks up the information in a pure form, because we don't have pure filters. I've seen people in a regression to the afterlife confirm much of what Newton wrote about. But, that's not all there is.
So, for what it's worth, here is my understanding. A soul group is a group of incarnating humans who work together to bring about certain experiences. Look closely around you...many of your family and friends are very likely in your soul group. Not necessarily all, but many. You continue to work together in the afterlife and while incarnate lifetime after lifetime. As you progress, or evolve, the group eventually changes. Just as on earth, some people "get it" faster than others. So, you can move to another group that is more in alignment with your current knowledge. Some can even be brought in who are not from earth, but have lived other existences, because they have expressed a desire to experience from a human point of view.
A disk (I-There, Higher Self, OverSoul, or whatever you want to call it) is a group
entitiy. It may or may not include individual souls from a particular soul group, such as your own. The number of individuals contributing to it is much larger than a soul group. It also acts as one being. Those in a soul group act as individuals with egos. The individuals that comprise your disk have each given up their personal identity (human ego) to become One Being. Not as easy as it sounds. You must be willing to give up the idea that you are
you. How many people do you know that are willing to merge themselves and lose who they are, with their identity becoming nothing more than a collection of experiences? Not many. That's one reason it is so difficult to graduate out of the earth system.
So, it's very complicated, and not as simple as just saying a soul group is the same thing as a disk. They exist on completely different levels of awareness. While a soul group, or certain individuals in it, may graduate together and merge as a disk, the disk will also include beings from other soul groups who have graduated. Some in your earth soul group may not even belong to your disk. Is that as clear as mud?