Updated for Odoo 16:

msg = 'All Users, please be advised ...'

env['mail.message'].create({
   'message_type': 'comment',
   'subtype_id': env.ref('mail.mt_comment').id,
   'model': 'mail.channel',
   'res_id': env.ref('mail.channel_all_employees').id,
   'body': msg,
    })

ORIGINAL POST:

You can do something like this:

msg = 'All Users, please be advised ...'

env['mail.message'].create({
   'message_type': 'comment',
   'subtype_id': env.ref('mail.mt_comment').id,
   'model': 'mail.channel',
   'channel_ids': [(6,0,[env.ref('mail.channel_all_employees').id])],
   'body': msg,
  })

This For Your Solution https://apps.odoo.com/apps/modules/17.0/rr_autopost_message_on_channel/

How to send to user? form code 

'res_id': env.ref('mail.channel_all_employees').id,