basically add a new type for scratch projects called "private projects" you can invite other people to Collab on it the same way you do with studios by writing their name into a box and hitting invite. you can also make permissions such as having it so a set person can only edit a specific set of sprites.
comment below if you want this to be added. the whole way it works is somewhat inspired Dreams. a PS4/PS5 game with collaboration as a feature. also heart as well if you want it to be added.