It's no problem, because:
1) joiners can wait outside the quest to speed up the process (the gong should work, iirc in rusted blades the gong works too if members not in quest)
2) in case they do enter, then the gong can be ignored and the things would be the same as they are now (i.e. wait for waves), so no change in this case
3) it would be even a good thing, because those who stay inside quest for xp shouldn't get xp faster, unless they are inside fighting room and help there