There are several places you can discuss abc notation, swap tunes, ask technical questions and find other abc users.
abcusers mail list
... the abcusers mail list is long established group for discussing abc notation and the software tools to handle it. Since several of the abc software developers subscribe, it is a great place to ask technical questions or start a discussion about issues arising from using the notation.
... the forums, at this site, are intended for discussion about abc, and sharing tunes and tips.
They are a good place to ask general questions about the notation.
The forums are disabled.
As well as usenet groups such rec.music.celtic and rec.music.folk, other places that are good for abc hints and tips, or swapping tunes, are community sites such as thesession.org (mainly Irish & Scottish music) or Mudcat (mainly folk song) where a lot of abc users hang out.
For more specific enquiries, see the contact page.