Has he joined the Democratic Party? NO. He is just playing political games with the process of the Democratic Party. His campaign spokesperson, Arianna Jones, said after he wins the Democratic nomination he will "respectfully" decline their nomination and run as an Independent. In fact, he has already started the process of getting enough signatures to get his name on the ballot as an Independent.
He's just playing political games with the Democratic Party. He has never been a Democrat, and still claims he is not a Democrat. Why run in the Democratic Party's primary then? To keep a REAL Democrat off the ballot in November -- someone who might drain enough Democratic votes to cost Sanders the election.
This is nothing less than a slap in the face to Democrats -- and Democrats need to remember it in case Sanders tries to run on the party's ticket in 2020. Do Democrats really want a person who refuses to join the party or wear its label to be elected as the leader of the party? I don't think so.
If Sanders wants to run for president in 2020, he should do so as an Independent.