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?
Best Answer
The penalty is being in difficult terrain
You're in difficult terrain for all creatures occupying the same space:
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:
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.