How can I message other scratchers?

You can go to their profile. Got to https://scratch.mit.edu/users/ and then add their username at the end.

There is however no way the private message.

Like powercon5 said, commenting on the users profile is usually the way Scratch users “message” each other, but I also would like to add one thing. Griffpatch made a chrome extension to make messaging and replying to comments easier.

I hope this was helpful.
-PlaneCrash