How to make Rules for chat app

[VincentCauch]

I am using flutter to make a chat app. I have group chat between two users, I don't know how to make rules so that only the two users can read and write in the group chat. This his how I store data on Firebase:

String date = DateTime.now() .millisecondsSinceEpoch .toString();
var ref = FirebaseFirestore.instance
.collection('messages')
.doc(groupChatId)
.collection(groupChatId)
.doc(date);

FirebaseFirestore.instance.runTransaction((transaction) async {
transaction.set(ref, {
"senderId": userId,
"receiverId": anotherId,
"timestamp": date,
'content': msg,
"type": 'text',
});
});

groupChatId is userId-anotherId

Thanks for any help.