I think it depends on how you're likely to query it. One option is certainly to have an "owner" id as part of each node, another to have a separate table which links each customer to the root node of their individual tree. The former can make it easier to identify the user from any give folder (since you don't need to interrogate the tree structure), whereas the latter can more easily allow for scenarios such as a tree belonging to a parent company who sees an entire tree with sub-entities each have a child tree beneath that.