I am also currently a leader for our research group. What we did was divide the work between us per chapter so that, while we focused on different areas of the research, we would all still have a general idea of what it was about as we went through each chapter. For example, for Chapter 1, we divided all the work between us four members. We did the same for the rest of the chapters. This way, everyone contributes fairly + is knowledgeable on the topic of your research. Keep in mind that when we divided the parts per chapter, we chose which we would work on. This is so as to ensure that the members work on parts that they know they are capable of doing.

However, I will say that if you have uncooperative members, you might have a difficult time executing this^{^}

Good luck on your research journey, OP!