@bot.event
async def on_member_join(member):
    with open('users.json', 'r') as f:
        users = json.load(f)

        await update_data(users, member)

        with open('users.json', 'w') as f:
            json.dump(users, f)

@bot.event
async def on_message(message):
    with open('users.json', 'r') as f:
        users = json.load(f)

        await update_data(users, message.author)
        await add_experience(users, message.author, 5)
        await level_up(users, message.author, message.channel)

        with open('users.json', 'w') as f:
            json.dump(users, f)

async def update_data(users, user):
    if not user.id in users:
        users[str(user.id)] = {}
        users[str(user.id)]['experience'] = 0
        users[str(user.id)]['level'] = 1

async def level_up(users, user, channel):
    experience = users[str(user.id)]['experience']
    lvl_start = users[str(user.id)]['level']
    lvl_end = int(experience ** (1/4))

    if lvl_start < lvl_end:
        await channel.send('{ctx.author.mention} has leveled up to level {}'.format(user.mention, lvl_end))
        users[str(user.id)]['level'] = lvl_end

async def add_experience(users, user, exp):
    users[str(user.id)]['experience'] += exp