H1B extension denied after RFE and upgrade to premium

H1 visa valid : 31 Dec 2018
I-94 : 10 Jan 2019

Employer A filed H1 extension on 20 Dec 2018 . Got RFE on Jan 10 . replied RFE on Jan 28 . Converted case to premium on Feb 21 and got denial on Mar 6. Case updated on website, yet to receive denial notice .

Meantime, Employer B filed H1 transfer on 28 Feb 2019.

  1. Can I Join Employer B?
  2. what are the options I have after denial .

You can join employer B but it is risky. Check with employer B to know if they are okay to make you join.

Now your H1B transfer cannot be approved without consular processing.

The best and safest option would be to leave USA and wait for transfer case approval outside USA.