I see that myofibrils (muscle cells) contain not one, but multiple nuclei.
Why is this so?
Do all the nuclei participate in cell division?
First Myofibrils are not muscle cells. Many myofibrils (essentially long strips of the muscle cell) make one muscle cell, which is known as a muscle fibre. And it's the muscle fibres that have multiple nuclei. And they have multiple nuclei because they were formed from the fusion of multiple myocytes. Why? Probably because the muscle cell is quite a large cell(because it is long) so multiple copies of DNA are required to produce the proteins required to maintain the muscle cell. Also muscle fibres do not divide, so no the nuclei do not participate in cell division. About other cells I am not sure. I am only aware that some cardiac muscle cells also have multiple nuclei, for similar reasons.
In the case of myofibrils this is the case because of the syncytial nature of that given structure. It means that in developing muscle tissue after a certain stage the cell divisions become somewhat different in that the final step, being separation of the daughter cells via invagination of cell membranes, does not happen. The nuclei though keep dividing after that. In the end what we have is a big membrane coated structure (basically a cell), with lots of nuclei in it.