What is the invitation process for Gatsby's parties in Chapter 3 of Fitzgerald's The Great Gatsby?

For the most part, there is no invitation process for
Gatsby's parties.  That's one of the points of the
parties.

People come from all over with no invitations. 
The word travels about the parties and the people of the Jazz Age are so eager to party
and drink illegal alcohol that they show up from all over.  The attendance at the
parties demonstrates the recklessness and carelessness and lack of consideration of the
people.  They come without knowing whether the host wants them there.  They don't care
if he wants them there or not.

This, of course, contrasts
with the attendance at Gatsby's funeral.  The contrast shows that the party-goers use
Gatsby like everyone else does. There is one exception, of course.  Gatsby does invite
Nick to join the party.  The process is that he just asks him to
come.