Do experience bonuses stack? Or Grocer hit point bonues?

Trainer bonuses apply to all units in the army, and they stack (which is why Henchmen can be so valuable - they can provide Trainer bonuses but they don't reduce experience like regular champions or Sov).  Potential trait bonuses apply just to the trait owner.

Guild Grocer is a variation of the new "Tough" ability you choose for your faction. AFAIK, you can have a global bonus of 30% maximum across your entire empire (so Tough and two Guild Grocers, or three Guild Grocers).  The +10% bonus applies to every unit in your empire, trained, Champion and Sov.  Individual bonuses (such as finite number of extra hitpoints, like a certain Fortress level bonus) still apply I think.  It's just that you can only have 3 stackable 10% HP bonuses at once.

In my game it stacks on a per city basis. i had 80% bonus in my last medium size game. Release version 1.00

What do you mean that it stacks on a per-city basis?  Do you mean that there is not a limit?  I guess the mechanics might have changed then.

I played Yithril in my last game and had nine towns with the guild grocer. My Tough bonus was at 100%. There seems to be no limit, as far as I can see. I also never noticed one during the beta. 

Okay, well I guess the comment about there being a limit of 30% was erroneous.  Unless it existed for some versions and not for others.  Though I never really leveled up many towns in beta 4 and 5.

I have seen this mentioned. How do I know what impact Sov and Champions have on experience? I don't see it in the manual or Hiergemanon? 

I think that base experience is divided up by number of champions for a given amount of XP.  I think creatures and troops only get a 1/2 of the base experience of the Champions.  I think troops get the same as with 1 Champion if there are no Champions.  Henchmen level up like Champions, and get XP based on base champion experience (and not troop experience) but they don't reduce the base champion experience at all like other Sovs/Champions do.

Yes, it is a pretty big oversight that it is not mentioned anywhere. But basically it means, that the XP gets divided by the amount of champions in the party. Two champions mean, that you only get half the XP, three champions mean one third, and so on. Regular units always get half the amount of XP your champions would get.

You play for 100 hours observing the experience gained from each battle...

Sorry to sound harsh , there are no easy ways to observe this, and it have taken me and some other beta testers hours to figure out the odds and ends of the current experience system.
We have several times asked that experience bonus attribute should be stated in the character details, I hope this gets in there along with critical damage multiplier.

That said, MOST bonuses stack, so its usually safer to assume it stacks than not. I don't think the Summoner bonuses stack though (several heroes with the "Summoner" trait), then again, just ignore those as they are rubbish anyways

Edit: Just wanted to state: I know for certain experience bonuses stack, 4 units with trainer 1 in the same group will increase XP gained by 40% to the whole group, if they are all heroes you will also experience the terribly hidden mechanic of XP Reduction. (divide amount of xp gained with amount of heroes in the stack).
Multiple grocers stack too, I don't quite like that feature though, this makes grocers extremely powerful in large empires, but that is personal preferences

Sincerely
~ Kongdej

Of course, having a henchman/hero with the Air school of magic (and therefore the Haste spell) is invaluable to giving your troops an edge in battle.  They can't gain experience if they're dead.

I do better with math than drawn out explanations.  So if I am understanding this correctly then in a given situation where you would be awarded 100 xp:

Champions:   

A lone champion gets 100xp.

2 Champions get 50 each or is the 100 reduced to 50 then divided so each gets 25?

Soldiers:

A lone soldier gets 100xp.

4 Soldiers get 25 each.

Depending on the answers above I can then figure out what should happen when champions and regular soldiers are combined and produce some tests to validate.

Champions get the full XP divided by # of champs in a group.

Non-champion units always gets 50% XP each.

For example you have an army with 3 champions and 3 non-champions and they win a battle which yields 100 XP.

Each champion gets 33 XP.

Each non-champion gets 50 XP.

Then, XP bonuses get applied on a per-unit basis.

1 champion would get 100 xp.

1 soldier would get 50 xp (they get less for some reason)

3 soldiers would all gain 50 xp

1 champion and 3 soldiers: The champion gets 100 xp, the soldiers gets 50 xp.

2 champions and 3 soldiers: The champions gets 50 xp, the soldiers get 25 xp.

3 champions and 3 soldiers: The champions gets 33 xp, the soldiers gets 17 (might differ a number or 2).

Basically the XP rewarded is always rewarded to everyone, but is divided by the amount of champions in the team, and then multiplied by specific multiplies, soldiers, beasts and other non-champions (Henchmen Included) gets half plus trait bonusses, and champions gets full plus trait bonusses.

Edit: Thank you Gaunathor

Sincerely
~ Kongdej

No, the Non-Champions would get 16XP (or 17 depending on how it gets rounded). Their XP is always based on how much the Champions get.

Henchmen are treated as Non-Champions for XP-purposes.

Thank you Gaunathor and Kongdej for your explanations.  I get it now.  

Geez, and no one referenced the stupid guide I've been putting together?  It has that information in the second 'concept', Experience.

Sorry. I thought of it, but it was faster for me to write it myself (highly unusual for me), instead of looking for the thread (sorry again, I haven't put it on my favourites yet). Next time I will. I promise.

Haven't noticed it, haven't been paying too much attention to FE forums lately.

Sincerely
~ Kongdej

1, 2, ... 9 soldiers would all get 50 exp each (assuming 0 - 1 champions)

1 Champion and 8 soldiers?

Champion gets 100 exp, soldiers get 50 exp.

2 Champions and 7 soldiers?

Champions get 50 exp each, while soldiers get 25 exp each.

OK, thanks everyone for answering the question. I note there is still some disagreement about what actually happens, but I think I get the gist of it.

Can I just inject at this point that it makes no sense at all?

Why would troops fighting with two or more heroes be less experienced than troops fighting with one? 

If the intention is, as I understand it, that there needs to be a mechanism to stop you just fighting with all heroes and not troops, then a more rational one should be found.