I agree with what everyone below said....
I just want to add that in 1st grade is when big birthday parties seem to go away. Girls start inviting a couple girls to sleep over rather than the entire class for a 2 hour party.... so some of that is natural, I think.
This is also when peer pressure is your friend. Her friends will take care of a LOT of her behaviors.... and let them. This is the age when you start to let her make her own choices.... but you change the way you talk to her about them and you let her find her OWN solutions.
SO -
When she says she argues with all her friends. Don't tell her not to argue... ask her why she argues? Ask her what her friends said. Get HER to talk and realize that she is either trying to be friends with girls who are very different from her OR she's being bossy and they are rebelling. Guide the conversation.... what would you do next time? How would that help? What do you like about Janie? etc etc.
I can tell you this..... I have a 12 year old and what I have noticed is they go in *waves*.... so 1st and 2nd grade is tough - they are figuring out who they are going to be and what their "clique" is. Finding friends. Excluding others.
I encouraged my daughter to exclude people nicely. just say "I don't want to play" practice how to say this so she doesn't sound like a brat. Practice inviting a girl who she WANTS to be friends with so she's comfortable.
This stage to me was night-marish. It's transition for both of you.
3rd - 4th/5th.... seemed to be a breeze. She's settled in with her friends, every so often there would be a "trade out" and one girl would leave the group and a new one would join, but for the most part they've work through their independence stage and they are getting good at using their new skills.
5th & 6th. Tricky. Girls get their period. They get boobs. They get hormonal and they cry for absolutely NO reason. They begin to "hate" things because they have NO control over their emotions. I hate my teacher "wail". I hate my siblings "wail". It's a ramped up version of 1st grade and what used to not start until 13....... Again.... just let her cry. I found during this stage (like when she was 2 or 3) if I REDUCED my emotion we were better off. You're the adult. You must act like it. And it's hard.
7th grade. Here is where we are. 7th grade started out a breeze. She'd worked through the hormonal stuff. Learned to use tampons. Learned to clean up the bathroom after aforementioned tampon use (ick). Probably has a COMPLETELY new set of friends. Make sure they are good ones. Let them all come over. Encourage YOUR house to be the sleep over house. Set some CLEAR boundaries with younger kids.
then...... BOYS. You didn't really see it coming. She'd been so well behaved and pleasant to be around. You almost remembered why you had her :-) Then, one day, she comes out of her room dazed, agitated and excited all at the same time. She's been texted. BY A BOY.
You've lost her. She's in her room, in her head or on her phone. As long as schoolwork and chores gets done.... let her be. Let her come to you, but reinforce the rules. Because she is now thinking with only 1/2 her brain. a year ago she could unload the dishwasher in about 5 minutes and everything was put away where it belongs. Yesterday I found a stoneware baking pan ON TOP OF MY PLATES in a completely different cupboard. and it takes her twice as long because her head is completely scattered.
Although annoying.... this phase is at least fun to watch!
Good luck