Both Your Sisters are fit

import random
#2D Array to store the users and passwords when the program is running
usersArray = []

def loadFile():
    usersFile = open("users.txt", "r")

    for line in usersFile:
        user = []
        user = line.split(",")

        user[len(user)-1] = user[len(user)-1].strip("\n")

        usersArray.append(user)

    usersFile.close()

def signUp():
    #Ask the user for their name
    firstName = input("Please enter your first name\n")
    surname = input("Please enter your surname\n")

    #Generate the first part of the username
    username = firstName + surname

    #Concatenate 3 random numbers at the end
    counter = 0
    while counter < 3:
        randomNumber = random.randint(0,9)
        username = username + str(randomNumber)
        counter = counter + 1

    #Ask for the password two times
    password = input("Please enter your password\n")
    password1 = input("Please re-enter your password\n")

    #Continue the program if the passwords match
    if password == password1:
        print("Passwords match")
    else:
        print("Passwords don't match")
        exit()

    print("Your username is " + username + " please make a note of this")

    user = []
    user.append(username)
    user.append(password)
    usersArray.append(user)
    print(usersArray)

    usersFile = open("users.txt", "w")

    for user in usersArray:
        lineForFile = ""

        for item in user:
            if item == user[len(user)-1]:
                lineForFile = lineForFile + item + "\n"
            else:
                lineForFile = lineForFile + item + ","

        print(lineForFile)
        usersFile.write(lineForFile)

    usersFile.close()

def login():

    Loggedin = False

    while not Loggedin:

        enteredUsername = input("Enter your username\n")
        enteredPassword = input("Enter your password\n")

        for user in usersArray:
            if enteredUsername == user[0] and enteredPassword == user[1]:
                print("login successful")
                Loggedin = True

        if not Loggedin:
            print("username and password incorrect")




'''Main Program'''

loadFile()

running = True

while running:

    menuChoice = input("Menu: \n1. Sign up\n2. Login\n")

    if menuChoice == "1":
        signUp()
    elif menuChoice == "2":
        login()

    answer = input("Do you want to run the program again? y/n\n")

    if answer == "y":
        running = True
    elif answer == "n":
        running = False