I have two good friends who just joined Scratch and I want to let them work on my game Club Giga, but how do I invite them to do that?

There's not actually a way that I know of to do that, but that would be something great to push for to give to the Scratch team.
At the moment however, unless you could visit their house, I'm pretty sure there's not a way to do it.
However, your friends might work on art, or certain scripts, then post a project from which you could take things to put in your backpack to then put intoyour project. Looking forward to seeing your project,
~klomaster10

They can remix your project, share it, and then you remix that back and forth. There is no real-time working together online, though.

Aww man…

You can have one user creating the sprites, another the backgrounds and a third the music. They would then export them when they have finished. These could then be imported into the main project for the Scratcher to program

What I have done is Emailed sprites and scripts and shared and unshared projects so my friend can get sprites into their project. Hope this helped!!!

What I do is I email sprites and scripts to my friends. Also, I shared projects so my friend can get sprites into their project, then unshared it so the general public Scratch community couldn't see it, they would just get the this project is not yet shared screen. Hope this helped!!!

Thx!