Document belonging to multiple realms

I have a use case where a user can create an object. The user has exclusive write access to this object, but has the ability to share it with one or more groups. Is it possible to have a partition key that allows the same object to be a part of multiple realms? For example, have a partition key in the format "ownerId=groups=[,,.

So far I have only been able to think of the workaround of making a copy of the object for each group, but that would be a significant inefficiency.

I formatted my example key poorly. See here: