Sorry for the delay. Looks like you could benefit from using our ‘tagging’ feature. This allows to tag items in a collection so you can have subsets of the collection that can be downloaded and synchronized. With tags, you could have one tag per each user and tag all items that they have access to. This will not split the collection in the cloud but will allow you to create on-device bundles that contain only items with a given tag.
You can find out more here: https://support.catchoom.com/customer/en/portal/articles/2870096-what-is-tag-management-for-your-collections-and-what-is-it-good-for