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?