No effect. Bone marrow donation requires only a small amount of the center of the bone so that there is no clinical effect on the density.
The central cavity.. Of bone, where the marrow is found, is distinct from the mineralized structural bone that most are familiar with. The extraction of marrow, from a donor, even significant quantities that would be used for donation in a stem cell transplant for an adult recipient, have no impact on bone mineral density. Donate away!