How do you use sprite groups?

I get the error message 'Enemy' object has no attribute '_Sprite__g' and I have no idea how to use the sprite groups. I'm trying to get them to spawn randomly across the x-axis only.

import pygame, os, random
pygame.init()
 
FPS=60

SCREEN = pygame.display.set_mode((400,500))
pygame.display.set_caption('caption')

x=50
y=450
vel = 3
width = 20
height = 20

class Player(object):
    def __init__(self,x,y,width,height):
        self.x = x
        self.y = y
        self.width = width
        self.height = height
        self.vel = 3
        self.image = pygame.Surface((width, height))
        self.image.fill((0,0,0))
    def draw(self,SCREEN):
        SCREEN.blit(self.image, (self.x,self.y))
class B():
    def __init__(self,x,y,radius, color):
        self.x = x
        self.y = y
        self.color = color
        self.radius = radius
        self.vel = vel
    def draw(self,SCREEN):
        pygame.draw.circle(SCREEN, self.color, (self.x,self.y),self.radius)
    
class Enemy(pygame.sprite.Sprite):
    def __init__(self):
        self.image = pygame.Surface((20,20))
        self.x = random.randrange (0,400)
        self.y = 500
        self.speed = random.randrange(1,3)
    def draw (self,SCREEN):
        SCREEN.blit(self.image,(self.x,self.y))
 
player = Player(x, y, width, height)
bullets = []
enemies = pygame.sprite.Group()
# Main loop
running = True
clock = pygame.time.Clock()
while running:
    clock.tick(FPS)

    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
    for bullet in bullets:
        if bullet.x < 450 and bullet.x >0:
            bullet.x += bullet.vel
        else:
            bullets.pop(bullets.index(bullet))
     
    keys = pygame.key.get_pressed()

    if keys[pygame.K_SPACE]:
        if len(bullets)< 5:
            bullets.append(B(round(player.x + player.width //2), round(player.y + player.height//2),3, (0,0,0)))
            
    if keys[pygame.K_w] and player.y > 30 - player.width - player.vel:
        player.y -= player.vel
    if keys[pygame.K_s] and player.y < 500 - player.width - player.vel:
        player.y += player.vel
 

    SCREEN.fill((190, 232, 220))
    player.draw(SCREEN)
    for bullet in bullets:
            bullet.draw(SCREEN)
    for i in range (8):
        e = Enemy()
        enemies.add(e)
    pygame.display.update()

pygame.quit()