According to the question Determining "level" of an NPC ally for purpose of budgeting encounter XP when one character helps another they can use the “help” action to confer Advantage on the roll. Additionally, when an entire group is pitted against another, the DM can do a “group check.” To make a group ability check:
Everyone in the group makes the ability
check. If at least half the group succeeds, the whole group succeeds.
Otherwise, the group fails.
These make sense for some activities – but for activities where combined effort is additive these approaches seem unsuitable. This would be particularly true for activities involving strength such as:
- Tug of War: A “group check” implies that a team of two could evenly beat a team of 100 depending upon the average strength of dozens of opponents. Even applying Disadvantage doesn’t seem to reasonably capture the odds of the situation.
- Carrying a couch up a flight of stairs or lifting a portcullis: For one person it might be impossible – but with five people the task could become trivial.
What are Adventurers League rules when combined effort should give cumulative advantages not just offer Advantage or treating both both sides as equal?
Additional Clarification:
Per the comment request below, the specific situation we are trying to solve is we are looking for rules that would cumulatively increase the PCs chance of success as each additional PC tries to lift a portcullis.
There is a portcullis in a pit that traps a PC away from the rest of the party. One or more other PCs may jump into the pit to help lift the portcullis. We were expecting that there might be a rule somewhere where every subsequent PC simply adds their strength to the effort so that, for example, a DC30 portcullis could be lifted, but haven’t seen anything. The use of the “Help” action doesn’t capture the additive help of additional people beyond one. Additionally, using the “Group Action” rule also doesn’t help if the DC is above the capability of the strongest person.
Best Answer
If the odds are that stacked in favor of either side, don't call for a roll
The Basic Rules pg. 58 and Player's Handbook both describe ability checks with the following prelude (emphasis mine):
When cumulative advantages stack that heavily against the player characters, the outcome is not uncertain; the player characters shouldn't succeed. So instead of letting them roll, just tell them that the task is impossible.
Similarly, if they have so many advantages stacked up on their side that it seems ridiculous that the PCs have a chance to fail, don't have them roll at all; just declare that they succeed. So in the case of your DC 30 portcullis trap, if you feel that the combined strength of the player characters is enough to lift the portcullis, then don't roll and instead declare that they succeed in lifting the thing.
In this way, you are only rolling for contests which are possible at all and don't have to worry about situations in which either PC success or failure is almost impossible to justify.
But what if their combined strength is only enough to justify a chance at success?
You may end up in situations, like the trap you describe, where the combined effort of every PC is enough to have a chance to succeed, but is not enough so that success is guaranteed. In this case, there aren't really many rules to help you. 5e is designed with simplicity at heart, so there are no rules for circumstantial bonuses. With that said, you are the DM and you have control.
In the same ability check section, the basic rules and PHB say:
You are in control of the DC, and you get to decide what it should be. You can even decide to change it on the fly to make your game work for you. We essentially solve the problem in reverse. Instead of looking for rules to modify the bonus to the roll, we leverage the power the rules give us as a DM to modify the target number.
You have every right to change the DC to whatever value you feel is most appropriate.
For example, 5 people lifting a portcullis might not be impossible, it might just be hard. So you can adjust the DC down from 30 to ~20 (based on the chart on typical difficulty classes in the basic rules and PHB).
You can even choose values between these markers. So, the task might be more than medium but not quite hard. In that case, you can decide "five people makes this a DC 17 or 18, not a 30". And then you can use either one roll from one of the players (with or without advantage at your discretion) or the group check rules now that the goal is much easier for everyone in the party to make.
This is the approach I use the most often in situations like these and I firmly believe that it's much less of a headache than trying to deal with rules for circumstantial bonuses.