This is tricky. I'm still trying to figure out how this would work. The thing is, if somebody wants to sign-up, you'll need to update the project, then everybody will have to re-download it. This could be solved if there was a way to send and recieve Shariables to other NetScratch projects, sorta like a server.
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"I'm having a headache with pictures...."
"An idea?"
"Uhh hun, uh hum!"