I have joined many online writers communities. The ones I found most useful were:
Authonomy – The Harper Collins site for aspiring writers. I attended a one day event they ran and got published as a result! I am now translated into 10 languages. What’s not to love!
Backspace – I was active in this community for years. Their articles were very helpful.
Goodreads – This site is mainly for readers, but you can register as a writer and run giveaways. I had 500 replies to a giveaway last Jan. I also enjoy some of the reading groups.
Preditors & Editors – Although I only used it a few times it opened my eyes as to what could happen to a writer.
International Thriller Writers is a useful genre group. They also run an amazing conference every July in New York. I attended one. It was fun!
Writing.ie – The Irish writing site with a constant flow of new articles on craft and great interviews. I attended a great PR for Writers day – free – as a result of this site recently.