I have not solved my problem yet. My ticket has assigned to Richie Lai since 3 days ago but no respond from him yet. I try to contact him in Twitter but inefficient.

Only Bittrex have the power to invite new members. I have an Enhanced account as well and have not been invited until today. It seems that they stopped inviting people for a while now that's why I gave you another option to follow up your concern aside from slack and sending a ticket. chiznitz has been answering queries here in the forum, so he might help.

Poloniex recently announced they will be taking action against unverified legacy accounts in a similar way to what Bittrex did.  The big difference being though that it's clearly outlined what will happen WAY ahead of time with plenty of notice for people to withdraw their coins if they wish.  And even after it takes effect people will still be able to remove their coins, just not trade them.

Unlike Bittrex which is a thieving piece of shit exchange that should have legal action taken against them soon.

As of this writing, the Bittrex team is still working on a mechanism for our enhance verified users to be invited to the Bittrex Slack channel.  Please check back later for further instructions.