It very much depends on what the grievance is about. It could be what WWW said, a bit of a lecture plus some kind of training or mentoring, coaching or performance management to modify their behaviour.
Or it could be a disciplinary situation, and they could end up with a warning of some description on their record as well, with a timescale after which it will be removed - 6 months is common.
Very much depends on what their actions have been.
In terms of how it will affect their career. Internally if their behaviour is not 'known' until this grievance is put forward, obviously it may colour the opinion of them among those that 'matter'. Longer term I would say if they change or continue to deliver results, that may well subside.
Based on my experience of these situations, I would expect the directors or whoever, to have a good knowledge of these people already and how they work. It may be that this grievance will give them an excuse to deal with them harshly. Or it may be that their impression of these people is fab, they deliver results, good at their job etc, and this type of behaviour is not regarded as a serious problem in which case long term impact will probably be low. I would be surprised if anyone important will be shocked and therefore impressions of them internally probably won't change hugely.
Externally, if there is a disciplinary ongoing or a warning on their record, it is something that can be asked by a future employer in a reference, and must be disclosed. So if that happens it is more likely to have an impact.
I hope that helps answer your question?
In terms of yourself, I am sure you understand that bringing a grievance is a stressful process. I would be inclined to put it in as soon as possible, then see how progress is and how you are feeling about it come February, and if you feel then that you'd rather stay out of the way and are finding it stressful, get the doctor to sign you off again. I would anticipate that that will be the case. I would not expect you back while this process is ongoing if I were involved.
Good luck.