Having read the sentence:

Oh JL, this is America - everyone is welcome!!

I could not understand:
  • What does "JL" exclamation mean?
  • Is it an abbreviation? of what?
Did you read this or hear it?

I think it's someone's initials.
Students: We have free audio pronunciation exercises.
Click on my link
(second line of my question)
He's talking to the previous poster, Jonathan Leffler
Oh, thanks, I see...
I thought that c alling a person by name (or its abbreviation)
is emphasized by puctuation marks (like comma, et al)

It is not so in English?
I touched this question here but have not got any definitive answer
Teachers: We supply a list of EFL job vacancies