Hey
09173140521 - You can think of the rule surrounding "each" sort of like the distributive property in math. If It's before the noun, like the example you gave, you're generally going to "distribute" the action (being at the park) to every girl is who is at the park. It's sort of like saying: "Rhea is at the park, Jenny is at the park, and Kathy is at the park" without having to list each out like that. My favorite example that makes this clear is "each of the girls brings her shoes to practice." Each girl has to bring her shoes individually, since it doesn't make sense that they lug and entire carton of shoes for everyone to share - each girl brings her own pair of shoes.
Also, with GMATclub threads like this one, it is often helpful to start a new thread for old topics like this one rather than continuing old ones since it's not incredibly likely that the users who started this thread are still on GMAT club over two years later!