Scratch

There are lots of ways to collab:

First, you should find a person to collab with. It would probably be better if it was someone you knew. Me and my friend funpanda made an account called funcollab, so you could make an account and make multiple or just make one and post it on both of your accounts.

To work on it you can possibly, send emails to each other (the sharing of emails should be done OUTSIDE of scratch) or visit each other and work on it.

You also need to decide a project to work on.

You can: Brainstorm ideas, or join Collab Camp 

Hope this helps