import vk_api

peer_id = 2000000000

vk_session = vk_api.VkApi('+71234567890', 'mypassword')
vk_session.auth()

vk = vk_session.get_api()

resp = vk.messages.getConversationMembers(peer_id=peer_id)

for i in resp['items']:
    print(i['member_id'])