give modpack creators the option to make their modpacks either public or private when uploading them. so if anyone want to create a modpack that is only supposed to be shared between friends, they can set it to private and give their friends a link so they have access to the modpack. maybe you can give other users editing access too so you can more easily collab when creating a modpack. have the modpack update for everyone if the author/anyone with editing access change anything.
i believe this would be a good feature for both modpack creators that want to do collabs and just groups of friends that just want to easily mod their games and play together.