Some friends and I collaborated over the net a long time ago, just for the fun of it. One of us started on something, sent the project to an FTP server, and the next person downloaded it, added something more, etc. And when someone decided a track was done, he put it online and started a new one.
These days Steinberg has a subscription based cloud service for Cubase intended for online collaboration, but I suspect it isn’t used much. At least it is hard to find info on how well it actually works. You can use it without being a Cubase user, but to me the whole thing looked a bit cumbersome. It could be interesting to try it, though, so If anyone would like to help test, I can see if I can create a project and invite collaborators.
I remember reading about a tool for realtime online jamming many years ago. I have forgotten what it was called, but the idea was to get around the problem of latency by increasing it to a set number of musical bars. So when you played something, the other musicians would hear it, say, 8 bars later, but still in sync with the beat. I thought that sounded like a lot of fun, but never got around to try it. EDIT: I suddenly remembered, it was called NINJAM.