A recent topic came up with similar issue.
Probably a better way than project sharing, is have the other user setup an account, then you can create for them a simple interface that controls the “thing” you want them too, on your host project/device, via Blynk Bridge.
Depending on the complexity of the interface you want to give them, they may or may not need a hardware device dedicated to your coding of such interface… or if just simple, webhooks and “simulated” hardware will also work.
Read more here if you chose, but please create a new topic of your own if you make somthing of your own and require assistance.
And if you want to keep it simple, and it is for family or someone you trust to NOT mess with it… just log them in with your account and they would have full simultaneous control as you do. Just teach them what NOT to touch… AKA the edit button