The ones I have done have all started out from this forum, or more accurately, the old version of this forum.
I did join the songwriters forum years ago in search of more opportunities, but it seemed a bit cliquey there and there were too many picky forum rules for my liking so I never took to it.
Personally, I'm not sure I could do it if I didn't have some kind of feel beforehand for what the other people are like.