To post a message to the list, just send an email to (you don't need to be subscribed): qubes-users@googlegroups.com
To subscribe to the list, send an empty mail to (you don't need a gmail account for that!): qubes-users+subscribe@googlegroups.com
I sent a blank email to qubes-users+subscribe@googlegroups.com am I suppose to get an auto reply that I have been subscribed?
Yes. (But it's always possible that Google recently changed something.) Did you check your spam folder? Are you able to subscribe to any other Google Group with that email address? AFAIK, there's nothing "special" about the qubes-users Google Group, so what you are experiencing is either a Google Groups problem, an email provider problem, or user error.
If it's not in your spam folder, try a different email account.
So how am I suppose to send emails as a subscribed user?
You just gave the answer: address your emails to qubes-users@googlegroups.com.