Rather than scaling HP, I'd suggest the following:

Scale the DPS of the particitipating players in such manner, that the marks live roughly the same time always independent of the player count attacking the mob. For example: B-Marks always need roughly 3 minutes, A-Marks 7 minutes and S-Marks 11 minutes to be killed after first engage and scale the DPS accordingly with players joining. This should be made with a minimum threshold, thus 1 player alone can't kill a S-Mark of course.