In my story, there are three characters who are in the "popular" clique at their school. I know that the three of them are mean, judgemental bullies, but I need to make them kind of different in some ways while still being similar enough to be following the leader of the clique. How else can I develop them so they are three different characters?
Give each one of them a background. Think about what they want in life and why. What are they good at? What are they afraid of? There are many questions like these you can ask for yourself in order to get to know them better to make them individuals.
I don't think you have to get into their histories. All you need is for them to have distinct personalities in current story time. These three are followers, so think of other followers you have known. Perhaps one is a suck-up who tries to please whoever is in charge. Another might be the sneaky sadist who pulls wing off flies and kicks animals when he or she thinks no one is watching, another could just be a sullen coward who hates (and fears) the world... I could come up with more, but really, tap into your own experiences.
I'm having one of the three girls be the leader. My main character is in that clique as well at first but she's not judgemental and cruel like the other three and, throughout the story, she breaks away from the clique.
The clique will always consist of pretty girls. The leader/Queen Bee of the group will always be the prettiest, most intelligent, and meanest one. There will always be at least 2 sidekicks who are also pretty and popular but not as pretty and popular as the leader. One will always be dumb and one will always be very insecure and seek self validation from the leader's approval. These dumb girl is in the clique b/c she makes the Queen seem smarter by comparison, they also both serve as willing servants to the Queen Bee's commands b/c the dumb girl is to dumb to question the Queen's authority/morals/ and intentions, and the insecure girl wants approval so bad that she'll do anything. The protagonist will almost always be a new girl who could potentially rise to the same status as the Queen Bee, since she is just a pretty and smart and could become as popular as the Queen bee. The Queen will invite her into the group in an attempt to prevent this from happening and also to boost her own profile. If you have time to kill and want to get familiar with clique dynamics, I would suggest watching some pretty entertaining films and clips and study the cliques in them, while they may seem formulaic and stereotypical I guarantee that after attending multiple high schools and universities that they are mostly alway true. Popular Cliques in Media include "The Plastics" from the film "Mean Girls" (2004), "The Heathers" from the film "Heathers" (1988), and "The Fashion Club" from the MTV cartoon Daria (1997-2001)A great example of a (semi) clique of hs girls who aren't the most popular but still in a clique none the less is the 2005 film "Pretty Persuasion." The film follows 3 girls who decide to frame their english/drama teacher for sexual harassment in order to make a name for themselves to jump-start their acting career. This is a great film to observe the sabotage and subterfuge that goes on in these cliques however this one is rated R for all the right reasons though so I'd suggest checking out it's rating before you watch it.