You can find collab groups pop up here sometimes, and also on the subReddits for WEBTOON, and webcomics in general. I was recently involved in one, and it was great fun.
Alternatively, if you have a number of webcomic mutuals, you could organise one yourself! I definitely want to do that in the future.
They become more frequent around big events such as July 4th, Halloween, Christmas, and so-on.