What happens when allies occupy the same space?

Question

According to "Moving Around Other Creatures" (PH p. 191 / SRD p. 92):

You can move through a nonhostile creature’s space. [...] another creature’s space is difficult terrain for you. Whether a creature is a friend or an enemy, you can’t willingly end your move in its space.

So it's possible for Alice to move into an ally's space, attack from there, then move out, the only penalty being that the ally's space counts as difficult terrain to enter.

If an enemy adjacent to the ally's space has readied a grapple against Alice and is successful, then Alice (unwillingly) ends her move in her ally's space.

Do allied characters in the same space suffer any negative effect?

Now the rest of Alice's allies do the same thing.

Is there a limit to the number of characters that can be stacked on a space?

What if Alice's ally was the one with the readied grapple?

Can you stack an ally on your space by readying a grapple to prevent them from leaving?

Answer 1

The penalty is being in difficult terrain

You're in difficult terrain for all creatures occupying the same space:

The space of another creature, whether hostile or not, also counts as difficult terrain.

The constraint is that creatures in the situation must attempt to move out of it.

The rules have a provision that generally avoids the stacking situation:

Whether a creature is a friend or an enemy, you can't willingly end your move in its space.

Grappling an unwilling ally will arrest them in your square: a clever edge case.

You can grapple your ally, providing they're unwilling, as grapple is a contest[1,2]. Sometimes one has to hold back their friend. The no-contest version is not a grapple, but is covered under the drag/lift/carry rules.

Cleverly grappling your ally to arrest their motion will work for a round. On the next round, both the grappler and the grappled characters must to attempt to move out of the square as long as they're both occupying it. Not attempting to move out of the square would mean they're willingly ending their move there, and that's not permitted per the rule cited above.

This is an example of the edge case of the rules and can be further manipulated into a broken situation that ends with a DM narrating when it stops.

The rules do not state a limit of the number of creatures in a space, because 5e rules are not a physics engine.

There are currently not specific rules about the number of creatures that can occupy a space. The provision is that they can't willingly do so in combat.

Making a large stack of creatures is similar to such endeavors as a peasant railgun or a hired army running through a teleportation circle.

Answer 2

Unfortunately, Mike Mearls is not the proper authority on these matters - although he does try his best to help. Since he openly admitted he could not recall the rules on the matter and gave an educated guess, Jeremy Crawford recalled them for him.

As per the intended RAW in PHB pg. 191, under "Moving Around Other Creatures":

"...you can’t willingly end your move in its space..."

Further explained by Jeremy Crawford in the tweet linked above:

"...you can't willingly end your move —any part of it— in another creature's space..."

So let's first clear up some confusion as to the phrasing of this rule.

• We know each creature gets a turn (1 turn) in the initiative each round; which starts being explained on PHB pg. 189.
• We know each creature may take 1 action (Attack action, Cast a Spell action, Ready action) and possibly 1 bonus action (there are several) during that turn, with details also beginning on PHB pg. 189.
• We know from PHB pg. 190 that you may also move an equal distance up to your speed, and can use as much or as little of your speed as you wish as well as breaking that movement in between various forms of travel (walking, climbing, jumping, etc) and that there is no 'Move Action'.
• We know from the next section on PHB pg. 190 that you can break up your movement in between attacks and other actions.

With what we know from the above clearly written rules, it then becomes clear that when Crawford says you can not end any part of your move (as detailed above) in a creature's occupied space, he is referring to your Speed-based movement, and is not solely referring to ending your turn as a whole, which can include any of the above listed. Your only choice is finishing your movement in an unoccupied space near the creature in question, attack your target of choice, then move back through (if you have movement remaining) the occupied space making sure you (once again) do not end any part of your movement in that creature occupied space. Then you can end your turn. Keep in mind that each time you move through the space, it is difficult terrain, and costs you extra movement unless you have the ability to ignore difficult terrain.

With this in mind... the only relevant question you have remaining perplexes me, and I think there are only 2 outcomes.

1. If Alice tries to move through an ally's space, but she is stopped by a Ready-action grapple from a hostile creature near the ally's space, then the Ready action you had in mind triggers (I am assuming?) the moment Alice is in range, interrupting her movement while she is still in her ally's space and effectively reducing her speed to 0, as per the Grappled condition.

Her movement has just been forcefully (unwillingly) stopped in another creature's space.

2. Alice tries to somehow move through a hostile creature's space but they Ready a grapple. In this case the Ready-action grapple would trigger when she got within attack range, because a grapple is always dependent on the Attack action, which you must forgo in place of a grapple. An enemy could never Ready a grapple to trigger when Alice steps into or onto their space.

• There are no rules that apply any penalty for this outcome.
• The space is difficult terrain anyway, so the addition of Alice to the space changes nothing.
• We know a 5-foot 'space' is generally 5 feet of 'effective creature threat', and not a humanoid/creature 5 feet wide. So roughly 10 humanoids could stand grappled together in a 5-foot space, which is 25 square feet.
• Stacking is another story entirely, and then you are dealing with distance/height in feet versus the creature's speed (which becomes 0 and so couldn't be tossed, thrown, or otherwise put on top of a grapple 'pile' higher than 1 foot tall).

Answer 3

Don't overthink this stuff, D&D 5 will tell you if there is anything special going on. Remember the 5-foot space is an abstract measure of the area a character controls, not that they physically occupy.

Do allied characters in the same space suffer any negative effect?

No.

Is there a limit to the number of characters that can be stacked on a space?

You mean apart from the physical one that a 5-foot space is 25 square feet or 125 cubic feet; this is a lot of human sized people - the current record for people in a telephone box is 14. A phone box is much smaller than a 5-foot space and if you are running combats with more than 14 people in them then they are going to take forever without trying to do stupid things like crowd everyone on one space.

Can you stack an ally on your space by readying a grapple to prevent them from leaving?

No, if you have readied a move to attack someone you are now hostile to them and they can't enter your space.

Answer 4

For allies: It would seem the rules as written interpretations can create for very silly requirements for getting allies to occupy the same space. I'd say the best way to handle it is allow allies to occupy the same 5 foot space and give advantage or disadvantage to checks as appropriate for the circumstance.

For Enemies: Just as an idea, for occupying an enemy's space I'd say it makes sense to be able to force yourself into their space provoking an attack of opportunity at advantage. Again circumstantially, it may make sense to occupy the same space, say if you have someone grappled or are pinning them.

Just remember to check with the others at the table to decide what the circumstance should bring about. Also, make sure the player taking the action knows the implications of their action before having them commit to it-- there's nothing more annoying than your clever plan/tactics unexpectedly backfire due to an unexpected ruling that you disagree with. Remember, the rules are a guideline, and the best way to have fun is to have things run in a way that everyone thinks is cool and fun.